Microbiology SBAs Flashcards
A 24 year-old Asian man presents with a persistent cough. A sputum sample is
taken and cultured on Lowenstein–Jensen medium, appearing as brown, granular
colonies after several weeks. The organism implicated is:
A Coxiella burnetti
B Streptococcus pneumoniae
C Mycobacterium tuberculosis
D Legionella pneumophilia
E Mycobacterium leprae
C Mycobacterium tuberculosis
This gentleman is most likely suffering from mycobacterium tuberculosis
which characteristically presents with a persistent cough, haemoptysis,
fever, night sweats and weight loss. Lowenstein–Jensen medium is a
growth medium used to culture Mycobacterium species at 37°C. The
most common indication for its use is to culture Mycobacterium tuberculosis
(C), where it appears as brown coffee-coloured (buff), granular
bread crumb-like colonies (rough) which often stick to the bottom of the
growth plate and are hard to remove (tough). This is often remembered
as ‘buff, rough and tough’. It usually takes approximately 4–6 weeks to
obtain these visible colonies, an important fact to remember when treating
patients. Another characteristic feature is the formation of serpentine
rods from chains of cells in smears. There are a few other important
points to remember about staining results for Mycobacterium tuberculosis.
They are classified as acid-fast bacteria, because they are resistant
to losing their colour during staining procedures. The Ziehl–Neelson
stain is the most common method used to stain this type of bacterium,
and they appear bright red against a blue background. The stain contains
carbofuchsin, a pink dye which binds to the unique mycolic acids
found in the mycobacterium cell wall. Another stain that can be used
for acid-fast bacilli is the auramine stain, which also binds to mycolic
acids to give a yellow fluorescence.
A 24-year-old HIV-positive Asian man presents with a cough. A Mantoux test
is performed. After 72 hours, the wheal diameter is measured at 5.8 mm. This
indicates:
A He has never been exposed to TB
B He has been exposed to TB
C He has had a BCG vaccination in the past
D He has latent TB which is now reactivated
E It is not possible to say
B He has been exposed to TB
The Mantoux test is a diagnostic test for tuberculosis. It consists of
an intradermal injection of 0.1 mL of purified protein derivative (PPD)
tuberculin, which is a glycerol extract of the bacillus. The diameter of
the induration that subsequently forms is read 48–72 hours later, but
one also needs to take into account the patient’s risk of being infected
with TB and of progression to disease if they were infected in interpreting
the result. The Centers for Disease Control and Prevention provide
the following classification for the skin test:
1 An induration of 5 mm or more is considered positive in:
• Patients with HIV
• A recent contact of a person with TB disease
• People with fibrotic changes on chest radiograph consistent
with prior TB
• Patients with organ transplants
• People who are immunosuppressed for other reasons (for example
taking the equivalent of >15 mg/day of prednisone for 1
month or longer)
2 An induration of 10 mm or more is considered positive in:
• Recent immigrants (
An 18-year-old university student develops a lower lobe pneumonia, with a
raised white cell count and CRP. A sputum culture reveals a Gram-positive
optochin-sensitive diplococcus. The most likely causative agent is:
A Staphylococcus aureus
B Streptococcus viridans
C Mycoplasma pneumoniae
D Streptococcus pneumoniae
E Haemophilus influenzae
D Streptococcus pneumoniae
It is useful to remember that streptococci can essentially be divided into
alpha haemolytic, beta haemolytic and non-haemolytic groups. Alpha
haemolytic streptococci can be further divided into Strep. pneumoniae
(D) and Strep. viridans (B) according to their optochin sensitivity
(amongst other factors). The beta haemolytic streptococci are further
classified according to Lancefield groups A, B, C, F and G. Finally the
non-haemolytic streptococci include the enterococci.
Optochin is an antibiotic used to differentiate Strep. pneumoniae
from other alpha haemolytic streptococci such as Strep. viridans. The
pneumococcus will typically produce a zone of inhibition around an
optochin disc, indicating that it is sensitive to the antibiotic, whereas
Strep. viridans is resistant to it so its growth will not be affected. This
can be remembered using the mnemonic ‘OVeR PS’ (Optochin – Viridans
Resistant, Pneumococci Sensitive). As the organism in the question is
optochin sensitive, the answer is (D).
A 58-year-old Caucasian alcoholic man presents to his GP with a history of
sudden
onset high fever, flu-like symptoms and, thick, blood stained sputum.
Achest x-ray is arranged which shows marked upper lobe cavitation. The most
likely causative agent is:
A Klebsiella pneumoniae
B Mycobacterium tuberculosis
C Staphylococcus aureus
D Moraxella catarrhalis
E Pnemocystis jirovecii
A Klebsiella pneumoniae
Klebsiella pneumoniae (A) is a Gram-negative rod-shaped bacillus that
can cause an atypical pneumonia, most frequently in alcoholics. It can
result in sudden, severe systemic upset in these patients, and the production
of thick, purulent and sometimes blood-stained sputum said to
resemble ‘red-currant jelly’. Haemoptysis occurs more frequently with
K. pneumoniae than with pneumonia caused by other bacteria.
Radiological features can include upper lobe consolidation, with marked
cavitation as described in the question. It is more likely to lead to
complications such as lung abscesses and empyemas than pneumonias
caused by Strep. pneumoniae.
A 27-year-old intravenous drug user presents with a 2-week history of fevers,
weight loss and a systolic murmur. The most likely causative agent is:
A Streptococcus viridans
B Candida albicans
C Staphylococcus aureus
D Streptococcus bovis
E Kingella
C Staphylococcus aureus
Infective endocarditis can be classified into two broad categories:
acute and sub-acute. Acute infective endocarditis is less common, and the most likely causative agent is Staphylococcus aureus (C). It can
affect both normal and abnormal valves, and can typically be found
in intravenous drug users, such as the patient described. The tricuspid
valve is most commonly affected in these cases, which can easily be
remembered as this is the first valve that the bacteria will encounter
following injection into a vein. Therefore, (C) is the correct answer in
this case.
A patient with shingles is treated with an anti-viral. The drug used is a guanosine
analogue and acts as a substrate for viral thymidine kinase. The most likely
drug she has been given is:
A Foscarnet
B Lamivudine
C Cidofovir
D Acyclovir
E Ganciclovir
D Acyclovir
Acyclovir (D) is a guanosine analogue that causes obligate chain termination
when it attaches to DNA. It is phosphorylated by the enzyme
thymidine kinase found in viruses, which is far more effective than
the cellular thymidine kinase for this process. This means that normal
cells which are not infected by the virus are not affected as much by acyclovir, as there is no viral thymidine kinase present. The acyclovir
monophosphate which then forms is further phosphorylated to a diphosphate
and then to a triphosphate by the cellular thymidine kinase. This
triphosphate potently inhibits viral DNA polymerase, leading to chain
termination. It is effective against the herpes viruses, for example herpes
simplex and herpes zoster which causes shingles.
According to the UK immunization schedule, which vaccine should be given to a
2-month-old baby who has already received DTaP (diptheria, tetanus, pertussis),
IPV (polio) and Hib (haemophilus influenzae type B) vaccines?
A Pneumococcus
B MMR
C Meningitis C
D BCG
E Hepatitis B
A Pneumococcus
The current UK immunization schedule is as follows:
• Two months: Hib/IPV/DTaP/PCV
• Three months: Hib/IPV/DTaP/Men C
• Four months: Hib/IPV/DTaP/PCV/Men C
• Twelve months: Hib/Men C
• Thirteen months: MMR/PCV
• Three years four months old or soon after: MMR/DTaP/IPV
• 13–18 years: Booster Diptheria and tetanus/IPV
A 24-year-old sexually active woman presents to her GP with dysuria. A urinary
tract infection is diagnosed. Which of the following is the most likely causative
agent?
A Enterobacter
B Escherichia coli
C Klebsiella pneumoniae
D Staphylococcus saphrophyticus
E Proteus mirabilis
B Escherichia coli
The most common cause of a urinary tract infection in all groups of
patients is Escherichia coli (B). Do not be misled by the fact that the
patient is a young, sexually active woman. The E. coli bacterium is a
lactose-fermenting Gram-negative rod. It has various properties that
aid its pathogenesis: a flagellum to enable it to move upstream, fimbrae
so that it can adhere to the urothelium, and haemolysin to form
pores in white blood cells. It also has a protective capsule called the
K-antigen. The other lactose fermenting organisms are Klebsiella and
Enterobacter, whilst non-lactose fermenting organisms include Proteus
and Pseudomonas. Lactose fermenting organisms turn MacConkey agar
pink, whereas non-lactose fermenters do not. Useful investigations for
urinary tract infections can include a urine dipstick to look for nitrites
and leukocytes, and urine cultures looking for a bactiuria of greater
than 105 colony forming units
A 44-year-old woman patient returns from her holiday in India with a 2-day
history of watery, offensive diarrhoea, bloating, excessive flatulence and abdominal
pain. The GP obtains a stool sample. Microscopy reveals a flagellate pearshaped
protozoan. The most likely organism implicated is:
A Bacillus cereus
B Salmonella enteritidis
C Giardia lamblia
D Entamoeba histolytica
E Cryptosporidium parvum
C Giardia lamblia
Giardia lamblia (C) is a flagellated protozoan parasite which causes
giardiasis. It attaches to the small bowel wall, but does not invade it.
If you can remember this fact, you will find it easier to remember that
it interferes with absorption, and so leads to the classic symptoms of
weight loss, flatulence, chronic diarrhoea and bloating, as in the patient
in this question. Because it does not invade the small bowel wall, the
diarrhoea is not bloody but it is watery. Microscopy of a stool sample
may show a pear-shaped protozoan. If you imagine a pear making you
feel very bloated, you will remember this fact which often crops up in questions! Very rarely, a string test may be done if other methods to
detect the parasites fail but there is still a high index of clinical suspicion.
A gelatine capsule attached to a long string is swallowed, with the
end of the string remaining outside the mouth and taped to the patient’s
cheek. It remains in place for about 4–6 hours, before the end is examined
under the microscope. Treatment of giardiasis is typically oral
metronidazole.
A 21-year-old medical student returns from her elective in India with a history
of abdominal cramps, vomiting, fevers and profuse, watery stools which she
describes as resembling ‘rice-water’. The GP obtains a stool sample. Analysis
reveals curved, comma shaped organisms that were shown to be oxidase positive.
The most likely organism implicated is:
A Hepatitis A
B Clostridium difficile
C Yersinia enterocolitica
D Campylobacter jejuni
E Vibrio cholerae
E Vibrio cholerae
Vibrio cholerae (E) causes profuse watery diarrhoea and vomiting. It can
in fact be one of the most rapidly fatal infectious illnesses if not treated,
because of the severe dehydration causing circulatory shock. The bacteria
produce a toxin which has an A and a B subunit. It is the A subunit
which activates a G protein and results in the production of cAMP,
which initiates the secretion of Na+, K+, Cl-, and HCO3
- into the small
intestine lumen. Most people only have a mild illness which simply resembles other diarrhoeal illnesses. Sometimes, as in this case, the diarrhoea
is profuse and is known colloquially as ‘rice-water’ stools because
of its appearance. The diagnosis is predominantly clinical, but if stool
culture is performed the classical appearance will be of curved shaped,
oxidase-positive organisms. You can remember this as the Cholera
Comma! Rehydration therapy forms the mainstay of treatment.
A 35-year-old HIV-positive man presents to his GP complaining of a general
feeling of tiredness, weight loss and night sweats. On examination there is hepatosplenomegaly
and hyperpigmentation of the skin. The most likely diagnosis is:
A Visceral leishmaniasis
B Cutaneous leishmaniasis
C Mucocutaneous leishmaniasis
D Malaria
E Schistosomiasis
A Visceral leishmaniasis
Leishmaniasis is transmitted by phlebotomine sandflies and occurs in
Africa, America and the Middle East. Visceral leishmaniasis (A) is also
known as ‘Kala-azar’, and the most common clinical features include
fever and splenomegaly. Hepatomegaly, skin hyperpigmentation and dry
warty skin occur less frequently, and bone marrow invasion can result in pancytopenia. It can be mistaken for malaria, which is dangerous as
it can be fatal if left untreated. L. donovani and L. infantum are thought
to cause the disease in Africa, Asia and Europe, whilst L. chagasi is
implicated in South America.
A 22-year-old student presents to accident and emergency with a raised, erythematous,
scaly ulcer on his forearm which has not been healing. On examination
he is also found to have lymphadenopathy. He gives a history of recently returning
from a 2-month trek in the rainforests of South America. Tissue is aspirated
from the margin of the ulcer, and the organism is cultured in Novy–MacNeal–
Nicolle medium. The organism implicated is:
A Toxoplasma gondii
B Treponema pallidum
C Leishmania dovani
D Leishmania major
E Leishmania braziliensis
D Leishmania major
The picture described is consistent with cutaneous leishmaniasis, the
most common form of leishmaniasis. An itchy, scaly papule develops
at the bite site and develops into a crusty ulcer with raised edges. Local
lymphadenopathy can also occur, but the lesion usually heals within
8 months leaving a depigmented scar called an oriental sore. The organisms
implicated are Leishmania major (D) and L. tropica. You can
remember this if you picture lots of skin lesions cropping up in travellers
from the ‘major tropics’! It is found in many countries, ranging
from South America to the Middle East. Diagnosis can be by Giemsa
staining of slit skin smears, or from tissue aspirated from the ulcer.
The organism can be cultured on Novy–Macneal–Nicolle medium as
described in the question.
A 35 year-old male clothing merchant has returned to the UK 2 weeks ago
from a visit home to Syria. A week later he presents with flu-like symptoms,
drenching
sweats and a recurring fever and is beginning to complain of lower
back pain. After further questioning, he mentioned that he worked on a farm
during his trip. He is successfully treated with oral doxycycline and gentamicin.
What is the most likely diagnosis?
A Malaria
B Tuberculosis
C Influenza
D Brucellosis
E Typhoid
D Brucellosis
The Brucella species are Gram-negative, rod shaped, intracellular bacteria
that cause a highly contagious zoonosis known as brucellosis (D).
The causative agent in cattle is B. abortis, but in dogs it is B. canis.
Infection in cattle can lead to miscarriages, hence the name ‘abortis’.
Infection is usually contracted from unsterilized milk, cheese or meat.
Clinical features of brucellosis can include a long history of undulating
fevers, arthralgia and myalgia, weight loss, fatigue, lymphadenopathy,
sacroilitis and depression. Many cases present as pyrexia of unknown
origin. Hepatomegaly and/or splenomegaly can sometimes be found on
examination.
A 50-year-old man has returned from hiking a segment of the Appalachian Trail
on the Eastern coast of the USA during the summer months. Ten days later he
presents to casualty with flu-like illness and a rash showing some central fading.
What is the most likely organism implicated?
A Herpes simplex
B Epstein–Barr virus
C Streptococcus pyogenes
D Treponema pallidum
E Borrelia burgdorferi
E Borrelia burgdorferi
Borrelia burgdorferi (E) is a Gram-negative bacterium that causes Lyme
disease. It is a spirochaete, which is the name for a group of bacteria
that are helically coiled in shape. Lyme disease is actually thought to
be the most common vector borne disease in England and Wales. It is
named after a town called Lyme in Connecticut, where the disease was
first seen. The vector is a tick called the Ixodes tick, which can be found
on deer and rodents.
Lyme disease is a multisystemic disorder which has three main stages: the
local stage, disseminated stage and a late stage. The local stage involves
a characteristic skin lesion called erythema chronicum migrans, usually
appearing 7–10 days after the initial infection. It usually starts off as a
red macule or papule, and approximately 1 week later expands to leave
a target appearance with an area of central fading. Other symptoms at
this stage are usually constitutional, such as a fever and headache. The
somewhat unusual features of the next stage can be remembered using
the word PEACH: Peripheral neuropathy, Erythema chronicum migrans
(persists in this stage), Arthritis, Cranial nerve palsies and Heart block.
Finally, the late stage can include persistent arthritis and chronic encephalitis.
Treatment is with oral antibiotics, usually doxycycline.
A 26-year-old squash player is admitted with a red, swollen left knee. He reports
no history of trauma. On examination he has a temperature of 38°C. A joint
aspirate is taken. What is the most likely causative organism?
A Neisseria gonorrhoeae
B Staphyloccocus aureus
C Haemophilus influenzae
D Streptococcus viridans
E Chlamydia trachomatis
A Neisseria gonorrhoeae
The most common cause of septic arthritis in young, sexually active
adults is Neisseria gonorrhoeae (A). A Gram-stain of this aspirate would
reveal Gram-negative diplococci. It is less likely for this organism to
lead to joint destruction than a staphylococcal arthritis. The two forms
of disseminated gonoccocal infection are the septic arthritis form (as
described in this case), and the bacteraemic form. Other clinical features
of the bacteraemic form might include a migratory polyarthralgia and a
vesicular or papular rash.