MCQ test practice bank Flashcards
What is its normal habitat?
- the upper respiratory tract.
Haemophilus influenzae may typically be isolated from the upper respiratory tract. In cases of meningitis, it gains access to the meninges via the bloodstream – the hematogenous route.
Immunization is useful for the prevention of:
- pneumococcal pneumonia in particularly susceptible patients.
- whooping cough.
- tuberculosis
The BCG vaccine protects against tuberculosis, and the DPT vaccine provides cover against the causative agents of diphtheria, whooping cough and tetanus. Haemophilus influenzae vaccines are available but do not provide protection against all the strains that can cause bronchopneumonia. Pneumococcal vaccines are now available.
What properties make this bacterium especially likely to cause prosthetic valve endocarditis?
- It is a skin commensal;
- It produces extracellular slime.
The coagulase-negative staphylococci are common skin commensals. As such, they may cause infections when introduced accidentally into surgical wounds. They frequently cause infections when devices are implanted. This is because many strains produce large amounts of extracellular slime. This acts as a type of microbial glue, enabling the bacteria to stick to the implants in a biofilm. This makes antibiotic treatment very difficult as antibacterial drugs penetrate biofilms very poorly. Consequently, when implants are infected, in many cases treatment will only be effective once the affected device is removed or replaced. Although biofilms make antibiotic therapy difficult and although the susceptibility of coagulase-negative staphylococci to antibiotics may be difficult to predict, this is not relevant in their role in causing endocarditis.
The following are examples of exogenous infection:
- cholera
- typhoid fever
Both typhoid fever and cholera are water-borne infections and so are exogenous. Dental caries results from the metabolism of sugars by the oral microbiota to produce acid that etches the enamel of teeth, producing carious lesions. ‘Honeymoon cystitis’ results from the introduction of Staphylococcus saprophyticus into the female urinary tract during sexual intercourse. Endocarditis is caused by many microorganisms, typically derived from the commensal microbiota.
Legionnaire’s disease is diagnosed by:
- a rising antibody titer over 14-21 days.
The diagnosis of Legionnaire’s disease relies upon a demonstration of a rising antibody titer over a 14-21 day period. A seven-day interval between tests is too short to demonstrate a clear difference in antibody titers between the acute and the convalescent samples. Although the causative agent, Legionella pneumophila, can be grown in artificial culture, it is fastidious and can be difficult to isolate. Urine is an inappropriate sample for culture examination in this disease.
The following pathogens have humans as their reservoir:
- Corynebacterium diphtheriae
- Salmonella Typhi
- Vibrio cholerae
Corynebacterium diphtheriae, Salmonella Typhi and Vibrio cholerae are all pathogens that have their reservoir in human populations. Salmonella Typhimurium is associated with animals and Clostridium tetani are found in soils (and the guts of many domesticated animals, but not humans).
The following survive(s) inside human cells and typically cause(s) systemic infection involving multiple organs known as enteric fever:
- Salmonella Typhi.
Salmonella Typhi is an intracellular parasite that causes enteric fever, a generalized infection that affects multiple organ systems. Salmonella Typhimurium produces a similar illness when it infects mice (as the name indicates) but typically causes gastroenteritis in humans; it is only invasive in people of fragile health. Shigella dysenteriae causes dysentery, sometimes known as the “bloody flux”. Vibrio cholerae is the cause of cholera. This illness is characterized by the production of a toxin in the gut that reverses the sodium pump, resulting in a massive fluid loss. Enterobacter cloacae is a hospital pathogen that causes opportunistic infections that are frequently difficult to treat because of the antibiotic resistances that these bacteria may express.
The following test(s) help in the diagnosis of Pneumocystis jiroveci pneumonia:
- bronchial lavage and microscopy.
Pneumocystis jiroveci pneumonia is one of the opportunistic infections that a transition from HIV infection to AIDS. The causative pathogen was once classified as a protist but is now recognized as a fungus. Bronchial lavage followed by microscopy is used to detect ‘cysts’. This may be helped by modifying the Ziehl-Neelsen or auramine-rhodamine staining protocols. It does not grow on conventional laboratory media.
What selective medium could be used to isolate this bacterium?
- neomycin fresh blood agar;
Gangrenous lesions are typically polymicrobial. In such cases, it is necessary to apply selective pressure to isolate the primary pathogen. In gangrene, the pathogen is an obligate anaerobe. As such, it is intrinsically resistant to the aminoglycoside antibiotic neomycin. Most other bacteria in gangrenous lesions are susceptible to this antibiotic. The inclusion of neomycin in fresh blood agar thus makes a good selective medium for this pathogen. Use of fresh blood in the agar is useful for another reason. The primary pathogen is hemolytic and so its colonies may easily be seen by a zone of clearing around each one growing on a medium containing fresh blood.
JC is 10 years old. He has just started boarding school and who rapidly developed pyrexia, headache, and convulsions. The onset of these symptoms was accompanied by the appearance of a petechial rash. A sample of his cerebrospinal fluid revealed 4.0 x 107 leukocytes per liter. The majority of these were polymorphs. No red cells were seen. The protein concentration was 2.3 g/l (normal range 0.14 to 0.45 g/l) and the sugar was 1.4 mmol/l. This compares with a blood sugar level of 4.6 mmol/l. A Gram stain revealed the presence of bacteria – intracellular Gram-negative diplococci within the cytoplasm of polymorphonuclear leukocytes (neutrophils). On incubating a chocolate plate inoculated with the CSF from this patient under 10% CO2, Isolate ‘E’ was made. Nothing grew when the CSF was cultured in air. The isolate is a Gram-negative diplococcus that was isolated under elevated CO2 is oxidase-positive.
- Neisseria meningitidis
Given the Gram-reaction, the requirement for an elevated level of carbon dioxide for artificial culture, the intracellular location and the fact that the patient is suffering a characteristic petechial rash, the identity of this pathogen is Neisseria meningitidis.
How may this bacterium have gained access to the bloodstream?
- Through DB’s prostate gland;
A transurethral resection is a very common operation for aging men. It is used to relieve the symptoms of prostatic enlargement and some of the prostate gland tissue is removed, leaving an open wound. It is via this damaged tissue that the bacterium has gained entry to the patient’s bloodstream in this case. Although DB’s urinary tract was the focus of this operation, there is nothing in the information given to suggest that he has a problem with his kidneys. Likewise, there is no suggestion that the patient’s bowel integrity was disrupted during the operation.
Given the information now available, the identity of this isolate is…
- a ‘viridans’ streptococcus
The fact that this bacterium is catalase-negative indicates that it is a streptococcus rather than a staphylococcus. The resistance to optochin and bile insolubility differentiates this “viridans” streptococcus from Streptococcus pneumoniae, another α-haemolytic streptococcus. Streptococcus pyogenes is β-haemolytic. During M’s evaluation, it was noted that her heart murmur, associated with a congenital valve defect, had changed. What is the explanation for this change? As the bacteria grow on damaged her heart valve, they stimulate the formation of “vegetations” by causing fibrin to be deposited. This causes further disruption of the blood flow through the valve, causing the murmur to change.
A culture of a fecal sample growing aerobically on MacConkey agar
The culture illustrated was made by streaking a feces sample onto MacConkey agar and incubating aerobically overnight. Two colony types are apparent, a large, red colony and a small, red colony.
The Gram film prepared from the larger of the colonies isolated from the fecal sample
This bacterium is a Gram-negative bacillus.
Growth on MacConkey agar results in large red colonies, indicating that this bacterium is a lactose fermenter.
The identity of this bacterium is:
- Escherichia coli
Tuberculosis:
- is a recognized cause of infertility in women.
- in children is often asymptomatic.
- in elderly patients is usually acquired from another individual with “open” tuberculosis.
- may be acquired by drinking unpasteurized milk.
Although this is very rare in countries with programmes for the screening and elimination of cattle infected by Mycobacterium bovis, tuberculosis may be a consequence of drinking unpasteurized milk. BCG vaccine is a live attenuated vaccine and is not heat killed. In a large study of 701 women with tuberculosis of the genital tract, infertility was the commonest presentation. Most infections in the elderly are reactivations of primary infection.
Microscopical examination of an appropriate specimen (e.g. sputum, swab, secretions) may permit the diagnosis of:
- Vincent’s angina.
- pneumococcal lobar pneumonia.
- tuberculosis
The presence of encapsulated lanceolate Gram-positive diplococci in purulent sputum is diagnostic of pneumococcal pneumonia, and the presence of acid-alcohol fast bacilli in specimens stained by the Ziehl Neelsen method is indicative of tuberculosis. Care must be taken when examining urine because the saprophyte Mycobacterium smegmatis may be confused with the pathogenic Mycobacterium tuberculosis. A Gram film of a throat swab showing the presence of fusiform bacilli associated with spirochaetes is diagnostic of Vincent’s angina. The presence of “atypical” monocytes in peripheral blood films may be suggestive of glandular fever, but they may be seen in other conditions as well. It is not possible to distinguish Bordetella pertussis from members of the human normal microbiota upon microscopy.
The following may be spread by infected blood products:
- hepatitis B virus
- hepatitis C virus
- human immunodeficiency virus
- Treponema pallidum
Hepatitis A virus spreads via the fecal-oral route. All of the other pathogens, including Treponema pallidum, the spirochaete that causes syphilis, may spread through infected blood or blood products.
The following may be part of the commensal microbiota of the vagina of a post-menopausal woman:
- Enterococcus faecium
- Escherichia coli
The commensal microbiota of a post-menopausal vagina resembles the fecal microbiota. Staphylococcus saprophyticus is a skin commensal that causes “honeymoon cystitis”, an infection of the urinary tract rather than the genital tract. Lactobacilli dominate the commensal microbiota of the vagina in women of reproductive age.
Bacteria responsible for community-acquired pneumonia include:
- Streptococcus pneumoniae
- Legionella pneumophila
- Mycoplasma pneumoniae
The commonest cause of community-acquired pneumonia is Streptococcus pneumoniae. Legionella pneumophila is an uncommon but important cause of his disease. Mycoplasma pneumoniae is another common cause of community-acquired pneumonia. Chlamydia trachomatis is a rare cause of hospital-acquired pneumonia in neonates. However, Chlamydia pneumoniae, a newly recognized organism, is increasingly recognized as a cause of community-acquired pneumonia. Pseudomonas aeruginosa lung infections are associated with ventilators used in hospitals to assist breathing.
Sore throat with fever:
- when recurrent is an indication for tonsillectomy.
Half or more cases of sore throat with fever are due to viral infection, and not Streptococcus pyogenes. In many cases, treatment may be deferred until the results of tests for streptococcal infection are available. A rise in antibody titer may not occur for two weeks or more following infection. Streptococcal sore throat is classically associated with rheumatic fever. Acute glomerulonephritis is also associated with streptococcal impetigo.
The following may be spread by the fecal-oral route:
- Shigella sonnei
- Vibrio cholerae
Vibrio cholerae and Shigella sonnei are spread via the fecal-oral route. Both rhinoviruses, which cause the common cold, and Neisseria meningitidis, the cause of meningitis, are spread through inhalation of infected droplets. As a venereal pathogen, Neisseria gonorrhoeae, the cause of gonorrhea, is spread during sexual intercourse.
ED is eight months old. She had suffered badly from eczema since birth. Because of this, her parents were advised not to have her vaccinated with the Hib vaccine at two months. She became unwell, and her family doctor arranged to admit her to the hospital because she was suffering from pyrexia and she was restless and off her feeds. A lumbar puncture was performed and Isolate ‘F’ was obtained upon the culture of the CSF. This bacterium is a pleomorphic Gram-negative bacillus that grows in the air but that is highly fastidious, requiring the presence of both the “X” and “V” growth factors - haem and NAD, respectively.
- Listeria monocytogenes
The features of this bacterium, particularly its nutritional requirements, indicate that it is Haemophilus influenzae. Were it to be subjected to strain typing, it would almost certainly belong to the Pittman type B.
Its identity may be confirmed by its dependence upon X and V growth factors (haem and NAD respectively). Alternatively, satellitism of colonies around Staphylococcus aureus growing on fresh blood agar will demonstrate nutritional dependence.
Babies are routinely protected from this infection by application of the Hib vaccine. In this case, however, vaccination had been contra-indicated, with disastrous consequences.
Recently, a molecular biological view of virulence and disease has arisen. Observations that link a gene with a virulence trait include:
- the gene in question must be translated.
- if disrupted, the gene encoding virulence converting the microbe from a virulent isolate to an avirulent form.
- introduction of the gene being tested into a strain of the same species that are not able to cause disease transforming the recipient strain to virulence.
- antibodies raised against the gene product being tested affording a degree of protection from illness.
To decide what genes code for factors that increase the ability of a microorganism to cause disease, the following should be demonstrated:
- the gene encoding the trait of interest should be present, transcribed and translated in a virulent strain;
- the gene encoding the trait of interest should NOT be present or should be silent in a strain that does not cause disease;
- disruption of the gene in a virulent strain should result in the formation of a strain that is incapable of causing disease;
- introduction of the gene into a strain that previously did not cause disease should transform the strain into one that does cause disease; N.B. some virulence traits may require the expression of more than one gene;
- the gene must be expressed during infection; antibodies raised against the gene product or the appropriate cell-mediated immunity should protect experimental subjects against disease.
Concerning dental caries:
- this may result in osteomyelitis.
- this may result in a life-threatening condition.
- this may regress in the early stages of the disease.
- this may result in the formation of a brain abscess.
Although Streptococcus mutans plays a central role in the initiation of dental caries by providing an attachment for many other bacteria that contribute to cavity formation, dental caries is a complex polymicrobial process. In the very early stages, dental caries may regress but in most cases, the treatment is to “drill and fill”. Life-threatening conditions including brain abscesses and endocarditis may result from dental caries. Untreated, another complication of dental caries may be osteomyelitis in the adjacent bones.
Compared with people in the wider community, people who are hospitalized are more likely to have urinary tract infections caused by:
- Proteus mirabilis.
- Staphylococcus aureus.
- Candida albicans.
Causes of urinary tract infections in hospital in-patients:
- Escherichia coli : 40%
- coagulase-negative staphylococci: 3%
- ‘other’ Gram-negative bacteria: 25%
- ‘other’ Gram-positive bacteria: 16%
- Candida albicans : 5%
- Proteus mirabilis : 11%
Causes of urinary tract infections in the wider community:
- Escherichia coli : 80%
- coagulase-negative staphylococci: 7%
- ‘other’ Gram-negative bacteria: 4%
- ‘other’ Gram-positive bacteria: 3%
- Proteus mirabilis : 6%
Thus, Candida albicans, Staphylococcus aureus, and Proteus mirabilis are over-represented as causes of urinary tract infections in hospital in-patients compared with people in the wider community.
Chlamydia:
- infections respond to tetracycline.
- cells contain DNA and RNA.
- infections are increasingly being diagnosed.
Chlamydial lung infections are characteristically bilateral. Although chlamydia is obligate intracellular parasites, they are bacteria, and they contain both DNA and RNA, unlike viruses which have only one type of nucleic acid. They lack cell walls, and so chlamydial infections cannot be treated with β-lactam antibiotics. However, they do respond to tetracycline therapy. Chlamydia is increasingly being recognized as human pathogens.
The gram film prepared from the smaller red colony isolated from the fecal sample
It is a Gram-positive coccus that tends to form chains.
Subsequent testing shows that this bacterium is not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase negative.
This isolate grows on MacConkey agar producing acid from the fermentation of lactose. It is thus considered to be a lactose fermenter. When this bacterium is plated onto fresh blood agar it does not alter the appearance of the blood cells.
The identity of this bacterium is:
- Enterococcus faecalis
The stomach is considered to be free of commensal microbiota. This is because the extreme acid pH at this site is inhibitory. The small intestine is increasingly colonized as it reaches the large intestine. The concentration of bile at the proximal end is inhibitory to many microbes.
The fecal microbiota has been estimated to have a metabolic capacity equal to that of the liver. Obligate anaerobic bacteria, such as those of the genus Bacteroides, outnumber the facultatively anaerobic coliforms, such as Escherichia coli (the commonest bacterium to grow aerobically from feces) by at least 100:1.
Diet may have a profound effect on the composition of the gut microbiota. Bottle-fed babies have a fecal microbiota that is entirely different from that of breastfed babies. The former resembles the adult fecal microbiota: the latter is predominantly bifidobacteria. The microbiota of bottle-fed babies breaks own proteins efficiently; the microbiota of breast-fed babies breaks down sugars. The smell and appearance of the feces reflect these differences!
Concerning sex bias in infections:
- men are more likely to suffer from leptospirosis than are women.
- boys are more likely to suffer from streptococcal skin infections than are girls.
Given the relative anatomies of the bowel and urinary tracts, it is women who are more likely to suffer from cystitis than are men but because infant boys are more likely to suffer from anatomical abnormalities that cause partial obstruction of the urinary tract, they are more likely to suffer from bladder infections than are infant girls. Boys are more likely to suffer skin abrasions leading to streptococcal skin infections than are girls. Occupational exposure increases the risk of leptospirosis in men compared with women; a disease associated with contaminated rat urine. There is no sex bias seen in meningococcal meningitis.
What feature of the bacterium is responsible for the clinical condition?
- Endotoxin encoded by the bacterial chromosome.
DB is suffering from endotoxic shock. Its symptoms include rigors, rapid heartbeat, and breathing and low blood pressure.
Although the genes that code for exotoxins may be located on the bacterial chromosome or on a variety of plasmids (and even as part of the genome of a bacteriophage), the genes responsible for the expression of endotoxin in Gram-negative bacteria are always located on the chromosome.
This is because endotoxin is an integral part of the Gram-negative outer membrane.
Before therapy for syphilis, a patient tested positive in both the RPR and TPPA tests. Assuming that therapy was successful, what pattern of test results would you predict after treatment?
- the RPR test will become negative but the TPPA test will remain positive.
The RPR test is a “non-specific” test. It relies upon the flocculation of cardiolipin antigen with antibodies in a positive serum. The test is enhanced by the addition of carbon particles, which aid the visualization of the flocculation. Although it is a non-specific test, it is useful in the following treatment, since the antibody titre declines on successful therapy. Thus, a patient who has been treated successfully will have a negative RPR test.
In the TPPA test, specific treponemal antigens are attached to gelatin beads. Unlike the RPR test, it is a specific test for treponemal infection. The treated beads are agglutinated by antibodies in a positive serum. The use of control beads in each test helps to identify false positive reactions. As a specific test of treponemal infection, once a patient has a positive TPPA test, this will remain positive for life.
Mycobacterium avium-intracellulare:
- is a common opportunistic bacterial pathogen in patients with AIDS.
- causes swollen lymph nodes in the neck in children.
Members of the Mycobacterium tuberculosis complex include Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium africanum and Mycobacterium microti, but not Mycobacterium avium intracellulare. It is the commonest cause of cervical lymphadenopathy in children in western countries and causes disseminated infections only in severely immunocompromised patients. It is a common opportunistic bacterial pathogen in patients suffering from AIDS. It is characterized by the production of non-pigmented, slow-growing colonies.
The gram film prepared from the greenish colony isolated from the throat swab
It is a Gram-positive coccus that tends to form chains.
Subsequent testing shows that this bacterium is not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase negative.
This isolate causes a change in the appearance of the blood surrounding the colony. It causes partial hemolysis, referred to as a-hemolysis, giving rise to a green halo around its colonies.
The identity of this bacterium is:
- a “viridans” streptococcus.
The oral cavity and throat have a rich and varied microbiota. There are marked differences between the different surfaces of the oral cavity. The bacteria that attach to the hard enamel of the teeth are different in type from those that colonize the soft tissues that are in intimate anatomical contact with the teeth.
There are many anaerobes that live in this area, and protists can also frequently be found. Both Gram-positive and Gram-negative bacteria are commonly seen in this area. The predominant bacteria are alpha-hemolytic streptococci. Because they are surrounded by a greenish halo when grown on blood agar they are sometimes known as ‘viridans streptococci’. Some people carry pathogenic bacteria as a part of the commensal microbiota of the throat. These include Streptococcus pneumoniae, Streptococcus pyogenes, and Neisseria meningitidis.
RS is a 46-year-old man with a history of chronic alcoholism. He was admitted to the hospital and gave a history of headache and vomiting. He was pyrexial, photophobic and had neck stiffness. Isolate ‘G’ was isolated from his CSF. It is a Gram-positive coccus that cannot release oxygen from hydrogen peroxide. It is also susceptible to optochin. Its colonies are surrounded by a greenish halo when growing on fresh blood agar.
What alterations have been made to the growth medium?
- α-haemolysis
Is this different from the bacteria that constitute the normal microbiota from sites within the body?
- Yes
The skin is often dry compared with other body surfaces and has a very high salt content. This makes it an osmotically hostile environment. As such, it is not suitable for organisms that are osmotically fragile. In consequence, most of the bacteria found in the skin microbiota are Gram-positive, with robust cell envelopes.
Elsewhere on the body, osmotic stress is not a problem and Gram-negative bacteria are just as likely to be found as are Gram-positive bacteria, although other factors may influence the relative distribution of different bacterial types.
Towards the end of her three-week stay, a 17-year-old student who visited England as part of a school exchange visit noticed that she was suffering from a vaginal discharge. On the following day, her left knee was swollen and red. It was very painful and was hot to touch.
Aspiration of the synovial fluid yielded a specimen of frank pus. Microscopic examination showed the presence of Gram-negative intracellular diplococci within polymorphonuclear leukocytes. At a later consultation, she said that shortly after arrival in England, she had had intercourse and that for a number of days before her joint problems she had noticed a purulent vaginal discharge. The culture of the knee aspirate is likely to yield:
- Neisseria gonorrhoeae.
Although Chlamydia trachomatis, Neisseria gonorrhoeae, and Ureaplasma urealyticum have all been associated with arthritis, the observation of Gram-negative diplococci in the knee aspirate indicates that this case is caused by Neisseria gonorrhoeae.
In non-specific urethritis:
- the most common causative bacterium is an obligate intracellular parasite.
- the condition may be associated with reactive arthritis.
- the most common causative bacterium may cause co-infection with Neisseria gonorrhoeae
- the condition may be associated with an eye infection.
The most common cause of non-specific urethritis, also known as non-gonococcal urethritis is Chlamydia trachomatis. Despite the alternative name for the infection caused by this bacterium, it commonly causes co-infection with Neisseria gonorrhoeae. Chlamydia trachomatis is an obligate intracellular parasite that cannot be grown on conventional laboratory media. It grows in McCoy cells and may be detected by DNA amplification. A common complication of non-specific urethritis is an eye infection, caused by autoinoculation with Chlamydia trachomatis. Much more rarely, a chlamydial infection may lead to Reiter’s syndrome, reactive arthritis associated with people with the HLA-B27 tissue type.
The following may be found as part of the commensal microbiota of the colon when cultivated in the air:
- Enterococcus faecium
- Escherichia coli
Corynebacteria and Staphylococcus saprohyticus are not found as commensals in the colon. Bacteroides spp are obligate anaerobes, and cannot be grown in air. Enterococcus faecium and Escherichia coli grow aerobically. The species names of both of these bacteria reflect their natural habitat: the gut. Remember, however, these species will be outnumbered by anaerobic bacteria by at least 100:1.
What is the identity of the causative agent?
- Clostridium perfringens
All of these pathogens can cause skin infections, although the obligately aerobic Pseudomonas aeruginosa requires the skin to be damaged, for example by a burn. Bacillus anthracis causes anthrax, the lesions of which are shiny and black. In this case, the patient is suffering from gas gangrene. This is caused by Clostridium perfringens .
The Nagler test is used to identify this bacterium. An egg-yolk plate is divided into two halves. Over one half is spread a specific antitoxin. The culture to be tested, together with the positive control, if available, is streaked across the plate, going from the untreated area of the plate to the area that is covered with antitoxin. The culture is then incubated anaerobically. Following incubation, lecithinase activity, caused by the action of the Clostridium perfringens a -toxin is seen as a precipitate in the medium on the side of the plate that did not receive the antitoxin, but not on the side that was treated.
Acute exacerbation of chronic bronchitis is associated with the presence of the following bacteria in the lower respiratory tract:
- Haemophilus influenzae
- Streptococcus pneumoniae
Haemophilus influenzae and Streptococcus pneumoniae are associated with acute exacerbations of bronchitis. Streptococcus pyogenes causes severe throat infections. Bordetella pertussis is the causative agent of whooping cough, and Staphylococcus aureusmay cause pneumonia.
The following may cause illness by intoxication:
- Staphylococcus aureus
- Bacillus cereus
- Aspergillus flavus
- Clostridium botulinum
- Claviceps purpurea
The fungi Aspergillus flavus and Claviceps purpurea produce aflatoxins. Aflatoxin, produced by Aspergillus flavus is a potent carcinogen; ergotamine, the product of Claviceps purpurea releases LSD when metabolized and is a potent hallucinogen. Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum all cause food poisonings. In the case of Staphylococcus aureus and Bacillus cereus, these are relatively trivial illnesses involving profuse vomiting for a limited period. Clostridium botulinum causes flaccid paralysis that carries a high risk of mortality.
The following is NOT one of Koch’s postulates:
- the pathogen causes antibodies to be raised in infected individuals.
Although these days one would expect antibodies to be raised against the causative organism in natural cases and in individuals infected experimentally, this requirement is not one of the postulates as stated by Robert Koch.
Koch’s postulates are:
- the microbe must be recovered from an experimentally infected host.
- the disease must be reproduced when a pure culture is introduced into a non-diseased, susceptible host.
- the microbe is present in every case of the disease.
- the microbe must be isolated from the diseased host and grown in pure culture.
It is not one of Koch’s postulates that a disease cannot be caused by more than one pathogen. There are examples when this does not occur e.g. pneumonia. The combination of the four postulates implies that a specific microbe causes a specific disease.
Clinical manifestations of osteomyelitis include:
- warmth around the affected area.
- redness around the affected area.
- systemic upset - fever, chills, night sweats.
- pain and swelling over the affected area.
In osteomyelitis, patients demonstrate reduced mobility of the affected area, rather than hypermobility.
The following bacteria may be isolated as part of the commensal microbiota of the oral cavity:
- Neisseria spp.
- Streptococcus mutans
- Streptococcus salivarius
- Moraxella spp.
Concerning the laboratory diagnosis of syphilis:
- once positive, the FTA(abs) test remains so for life.
- once positive, the TPPA test remains so for life.
- screening of donated blood is routine in the United Kingdom to prevent the horizontal spread of the disease.
Treponema pallidum will not grow on conventional laboratory media. While animals may be used for the propagation of Treponema pallidum in the laboratory, the usual model is the eyes or testes of rabbits; the nine-banded armadillo is used for the propagation of Mycobacterium leprae. Screening of blood for syphilis is used for all donations in the United Kingdom . The diagnosis of syphilis typically relies on serological testing. The tests may be specific for treponemal antigens or non-specific, seeking antibodies to antigens other than those expressed by Treponema pallidum. Once positive, the specific antigen tests such as FTA(abs), the fluorescent treponemal antibody absorption test, and the TPPA - Treponema pallidum particle agglutination test - remain positive for life. Non-specific tests such as the rapid plasma reagin (RPR) test become negative following successful treatment of the infection. Such non-specific tests are useful in the monitoring of therapy.
Osteomyelitis may be caused by:
- Staphylococcus aureus
- Streptococcus pyogenes
- Klebsiella pneumoniae
- Mycobacterium tuberculosis
All of the pathogens listed may cause osteomyelitis apart from Bordetella pertussis, the cause of whooping cough.
The gut microbiota provides protection against infection caused by:
- Clostridium difficile
Clostridium difficile causes antibiotic-associated diarrhea. Following administration of an antibiotic, particularly one that is excreted via the gall bladder, the commensal microbiota of the gut becomes disturbed. This provides toxigenic strains of Clostridium difficile with the opportunity to flourish and cause disease. This may range from relatively mild diarrhea to the potentially fatal pseudomembranous colitis and rupture of the colon. This illustrates the role of the commensal microbiota in preventing disease.
Clostridium acetobutylicum is used in the production of acetone from glucose. It was Chaim Weizmann’s work with this bacterium that led to the establishment after the Second World War of Israel as an independent state. Clostridium botulinum is the cause of a flaccid and often fatal flaccid paralysis known as botulism. Clostridium perfringens may be isolated as a gut commensal and Clostridium tetani, a soil bacterium, causes tetanus, a disease in which muscles go into spasm.
Legionnaire’s disease:
- often presents as atypical pneumonia characterized by high fever and confusion.
- is named after a convention of First World War veterans.
- causes deaths in the UK.
Legionnaire’s is so named after the 58th annual convocation of the First World War veterans, held in the Hotel Bellevue, Stratford in Philadelphia in 1976. Over 200 delegates acquired severe pneumonia and 30 died. The causative agent is a Gram-negative cocco-bacillus, but the treatment of choice is erythromycin, an agent that is typically only active against Gram-positive bacteria. It is a fastidious bacterium, and will only grow on media rich in iron and cysteine, and containing activated charcoal, used to adsorb its toxic metabolic products. The disease is derived from inhalation of infected water droplets and is typically associated with dirty shower heads or Jacuzzis, not conventional bath taps. This is because taps do not produce air-borne water droplets of a size that will facilitate infection of the alveoli.
Pneumonia caused by Streptococcus pneumoniae:
- is a complication of splenectomy.
- is usually caused by bacteria that can be isolated from the respiratory tract of healthy adults and children.
- is defined as a bacterial infection of the lung parenchyma.
Definitive diagnosis of streptococcal pneumonia relies upon the detection of the bacterium in blood cultures. In the United Kingdom, the vast majority of strains remain sensitive to penicillin.
In acute uncomplicated urinary tract infection:
- numerous white blood cells are present in the urine.
- E. coli is the most frequently encountered pathogen.
- the bacterial count is usually >105 organisms/ ml of urine.
Surveys have shown that overall about 80% of UTIs are caused by E. coli and infections are seen more frequently in women. The criterion of >105 organisms/ml is a reliable measure of significant bacteruria which with significant pyuria (>10 WBC/cu.mm) invariably indicate an infection. However, counts of 104/ml and even 103/ml may be significant if pyuria and symptoms are present.
The following may cause a flaccid paralysis:
- Clostridium botulinum
Clostridium botulinum causes flaccid paralysis that carries a high risk of mortality. Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum all cause food poisonings. In the case of Staphylococcus aureus and Bacillus cereus, these are relatively trivial illnesses involving profuse vomiting for a limited period. The fungi Aspergillus flavus and Claviceps purpurea produce aflatoxins. Aflatoxin, produced by Aspergillus flavus is a potent carcinogen; ergotamine, the product of Claviceps purpurea releases LSD when metabolized and is a potent hallucinogen.
Gram film prepared from the only colony isolated from a vaginal swab taken from a woman of reproductive age
It is a Gram-positive bacillus. It typically stains weakly and unevenly.
Subsequent testing shows that this bacterium is not motile and is also not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase negative.
The identity of this bacterium is:
- a lactobacillus (Döderline bacillus)
The following microbes do not fit Koch’s postulate requiring an artificial culture of the pathogen:
- Mycobacterium leprae
- Treponema pallidum
Treponema pallidum and Mycobacterium leprae cannot be cultured on laboratory media, and thus cannot fulfill the requirement for artificial culture in Koch’s postulates.
The following may be spread by the respiratory route:
- rhinoviruses
- Neisseria meningitidis
Both rhinoviruses, which cause the common cold, and Neisseria meningitidis, the cause of meningitis, are spread through inhalation of infected droplets. Vibrio cholerae and Shigella sonnei are spread via the fecal-oral route. As a venereal pathogen, Neisseria gonorrhoeae, the cause of gonorrhoea, is spread during sexual intercourse.
The following bacteria can cause serious respiratory disease in the community:
- Staphylococcus aureus.
In acute osteomyelitis, holes are drilled into the bone:
- to provide a specimen for the laboratory.
- to relieve pressure in the bone.
- to debride the affected bone.
- to reduce pain for the patient.
Drilling holes in the bone will release the pressure caused by the accumulation of pus in the bone. This will allow the affected bone to be cleaned (debrided) and will provide easy access to a sample of pus for pathological examination. Most importantly for the patient, however, releasing the pressure will give instantaneous pain relief.
Life-threatening infections caused by commensal bacteria include:
- endocarditis
- gas gangrene
- septicaemia
- peritonitis
Endocarditis may be caused either by oral bacteria settling on damaged heart valves or from skin bacteria introduced into the circulation during operations to replace damaged heart valves. Injecting drug users may suffer a particularly serious form of endocarditis caused by Staphylococcus aureus.
Gas gangrene is caused by Clostridium perfringens, a gut commensal. Peritonitis is a condition in which the gut ruptures, permitting access of the gut commensals to the peritoneal cavity. Meningitis caused by commensal bacteria may occur when, for example, plastic ‘shunts’ are placed in the brain to relieve the symptoms of hydrocephalus. In such cases, the likely causative bacterium will be a coagulase-negative staphylococcus.
Mycobacterium tuberculosis:
- when cultivated in vitro has complex growth requirements.
Mycobacteria are non-sporing rods that are classified with the Gram-positive bacteria on the basis of their cell wall architecture. However, the hydrophobic cell wall of Mycobacterium spp (up to 60% of the dry weight is lipid) precludes staining by Gram’s method. Other staining procedures e.g. Ziehl-Neelsen are therefore used. Mycobacterium tuberculosis is an obligate aerobe - even a small reduction in oxygen tension will hamper the growth of the bacterium. It is resistant to drying (it may survive in dried sputum for up to six months). Even under optimal conditions, Mycobacterium tuberculosis divides only every 18-24 hours. Thus it can take from six to eight weeks for a laboratory culture to grow. More rapid, semi-automated methods have reduced this time and PCR is often used to provide a rapid provisional diagnosis.
Culture, growing on fresh blood agar, of a high vaginal swab from a post-menopausal
The culture illustrated was made by streaking a vaginal swab from a post-menopausal lady onto fresh blood agar and incubating aerobically overnight. Two colony types are apparent, a large, buff colony and a small, buff colony.
Gram film prepared from a vaginal swab from a post-menopausal lady
In the Gram film, two different types of cell can be seen; one is a Gram-negative bacillus and the other is a Gram-positive coccus.
The Gram-positive cocci form short chains. The Gram-negative bacilli show no particular arrangement.
Gram film prepared from the larger of the colonies isolated from the post-menopausal vaginal swab
This bacterium is a Gram-negative bacillus.
Growth on MacConkey agar results in large red colonies, indicating that this bacterium is a lactose fermenter.
The identity of this bacterium is:
- Escherichia coli
In the acute sore throat:
- the disease is more common in the winter than summer.
- the disease is very rarely acquired from contaminated food.
Most sore throats have a viral etiology, but these cannot always be distinguished clinically from streptococcal sore throats. Sore throats tend to be more common in winter than in summer. Food is, admittedly very rarely, a vector for throat infections.
Disease due to Mycoplasma pneumoniae:
- is treated with tetracycline or a macrolide antibiotic.
- occurs in epidemics.
Mycoplasma pneumoniae infections show a periodicity of about four years. Because these bacteria lack cell walls, they cannot be treated with β-lactam antibiotics such as the penicillins or cephalosporins. Instead, they succumb to treatment with tetracyclines or macrolides. These latter antibiotics inhibit bacterial protein synthesis.
The Nagler reaction:
- is carried out on egg yolk agar.
- is used to detect lecithinase production.
- is used to identify Clostridium perfringens.
The α toxin of Clostridium perfringens is a lecithinase, causing a white precipitate in egg-yolk medium, a rich source of lecithin. In the Nagler reaction, its activity is inhibited specifically by the presence on half the plate of anti-toxin.
The commensal microbiota within the oral cavity is influenced to a major extent by:
- eruption of the first dentition.
- tetracycline
- amoxicillin
The use of antibiotics, particularly those with a broad spectrum of activity, have a profound effect on the human commensal microbiota. Microbes have evolved to adhere to particular types of surface, and so when teeth first appear, this presents a new type of surface to which microbes become attached. This, in turn, leads to the mouth becoming colonized with a new array of microbes. It is the enamel on teeth that is important for this adhesion, and not whether the tooth is a first set or second set. False teeth provide yet another surface, providing for a different array of colonizers. Diet has little effect on the resident microbiota of the mouth, but it may influence the transient microbiota.
Endocarditis following a dental extraction
MT has a congenital heart valve defect and has recently undergone a dental extraction. Since this procedure, she has complained of excessive tiredness. Examination revealed low-grade pyrexia, changed heart murmur, anemia, and microscopic haematuria. ‘Splinter hemorrhages’ were apparent under the fingernails. M had not received any antibiotics recently. Three sets of blood cultures were taken over a 24-hour period. Two of the three sets yielded Isolate ‘D’ on subculture. It is a Gram-positive coccus that fails to release oxygen from hydrogen peroxide. Its small colonies are also surrounded by a greenish halo when growing on fresh blood agar.
The change in color of the blood in the agar indicates that the bacterium is:
- α-haemolytic
This bacterium happens to be catalase-negative but this does not have any influence on the appearance of its colonies on fresh blood agar.
The greenish halo that surrounds each colony of this bacterium is indicative of partial hemolysis – also known as α-hemolysis.
The green appearance around colonies is even more apparent when this bacterium is grown on heated blood agar but, in this case, the effect cannot be due to hemolysis, since heating the blood in HBA destroys the red blood cells that are the targets of hemolysis. The green color is thought to be the result of the partial breakdown of haem to give biliverdin, a green pigment.
Complete hemolysis, in which bacterial colonies are surrounded by a zone of complete clearing, is referred to as β-hemolysis. To tell the difference between α- and β-hemolysis, just remember that “beta is complete”.
Gram film prepared from the large flat colony isolated from the skin swab
It is a Gram-positive bacillus. It forms groups that are said to resemble ‘Chinese letters’. Following liquid culture, these bacilli may be seen lined up like fence-posts. This arrangement is said to resemble a palisade.
Subsequent testing shows that this bacterium is capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase positive.
The identity of this bacterium is:
- Corynebacterium sp.
The skin is a relatively dry environment. It also has an acid pH. It is thus inhospitable for many microbes. Most of the microbiota of the skin are Gram-positive bacteria such as the coagulase-negative staphylococci and corynebacteria. They can withstand drying better than other microbes.
There are several species of coagulase-negative staphylococci and they are not often differentiated to species level. This bacterium is thus most accurately described as a coagulase-negative staphylococcus. Given the origin of the culture in this case, however, if it were to be identified to species level it would most likely be Staphylococcus epidermidis.
Anaerobic bacteria can live within the pores of the skin. Propionibacterium acnes is an example. It metabolizes sebum, releasing free fatty acids. These enhance the antimicrobial environment provided by the skin.
The eye has a specialized type of skin. It is kept clean by the washing action and inhibitory factors found in tears. In consequence, few microbes are found on the eye. Some people carry coryneform bacteria on their conjunctivae.
Is this bacterium likely to be soluble in bile?
- yes
The following may cause vivid hallucinations:
- Claviceps purpurea
Claviceps purpurea produces the mycotoxin ergotamine that releases LSD when metabolized. This is a potent hallucinogen. Aflatoxin, produced by Aspergillus flavus, is a potent carcinogen. Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum all cause food poisonings. In the case of Staphylococcus aureus and Bacillus cereus, these are relatively trivial illnesses involving profuse vomiting for a limited period. Clostridium botulinum causes flaccid paralysis that carries a high risk of mortality.
Endotoxin:
- cause thrombosis.
- causes fever.
- causes vascular collapse.
- equates to the lipid A component of lipopolysaccharide.
Endotoxin consists of the lipid A component lipopolysaccharide from the outer cell membrane of Gram-negative pathogens. They are especially dangerous molecules to the host, causing a wide range of damage including vascular collapse, fever, hypoglycemia, thrombosis, iron depletion, reduced blood pressure.
The Lübeck disaster in 1926, which resulted in the deaths of 76 children, was associated with the administration of a virulent strain of:
- Mycobacterium tuberculosis
The Lübeck disaster was caused when a virulent strain of Mycobacterium tuberculosis rather than the vaccine strain was administered to 249 babies, resulting in the deaths of 76 children. Hepatitis B virus contaminated a vaccine used to protect American service personnel from yellow fever in 1942. While there are vaccines that afford some protection against leprosy, they are not accepted universally. There is, as yet, no vaccine against Plasmodium vivax.
The following may be spread venereally:
- Neisseria gonorrhoeae
In addition to being catalase-negative and α-hemolytic, this Gram-positive coccus is also resistant to the antibiotic optochin. Is it soluble in a solution of bile salts?
- No
Being resistant to optochin, the isolate will also be insoluble in a solution of bile salts.
Culture of a throat swab growing on fresh blood agar
The culture illustrated was made by streaking a throat swab onto fresh blood agar and incubating aerobically overnight. Two colony types are apparent, a largish, buff-colored colony and a very small colony that has caused the surrounding medium to turn green.
Gram film prepared from the throat swab
In the Gram film, two different types of cell can be seen; both are round. One is Gram-positive, the other Gram-negative. Thus, this film contains both Gram-positive and Gram-negative cocci.
The Gram-positive cocci are found in chains; the Gram-negative cocci are generally arranged in pairs and are said to be ‘diplococci’.
Gram film prepared from the largish, buff colony isolated from the throat swab
This bacterium is a Gram-negative coccus that is generally arranged in pairs and is said to be ‘ diplococci’.
Subsequent testing shows that this bacterium is capable of turning a 1% solution of tetra methyl-para-phenylene diamine dihydrochloride, TMPPD, deep purple within five seconds of exposure to the reagent. It is thus considered to be oxidase positive.
The identity of this bacterium is:
- a member of the family Neisseriaceae
Gram film prepared from the flat, dry-looking colony isolated from the nasal swab.
It is a Gram-positive bacillus. It forms groups that are said to resemble ‘Chinese letters’. Following liquid culture, these bacilli may be seen lined up like fence-posts. This arrangement is said to resemble a palisade.
Subsequent testing shows that this bacterium is capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase positive.
The identity of this bacterium is:
- Corynebacterium sp.
Many of the microbes that we encounter enter through the nose. Indeed, the nose has elaborate anatomy and nasal hairs, evolved to prevent potentially dangerous microbes from entering our lungs.
The nasal microbiota resembles that of the skin in many respects. However, the air that we exhale is moist, and this makes the nose a more hospitable environment for Staphylococcus aureus, a bacterium that can cause serious infections as well as being a commensal.
Gram film prepared from the smaller red colony isolated from the post-menopausal vaginal swab
It is a Gram-positive coccus that tends to form chains.
Subsequent testing shows that this bacterium is not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase negative.
This isolate grows on MacConkey agar producing acid from the fermentation of lactose. It is thus considered to be a lactose fermenter. When this bacterium is plated onto fresh blood agar it does not alter the appearance of the blood cells.
The identity of this bacterium is:
- Enterococcus faecalis.
The vaginal microbiota changes dramatically through life. When a baby girl is born, she still carries her mother’s hormones for a short while, and the vaginal microbiota is dominated by lactobacilli, sometimes called “Döderlein” bacilli. As the maternal hormones decline, the microbiota changes to resemble that of the bowel. However, with the onset of puberty reproductive hormones are produced, and the lactobacilli return. At the menopause, the microbiota reverts to resemble that of the bowel once more. Upon aerobic culture, E. coli and enterococci are the predominant species.
