Microbiology Flashcards
What are prion proteins?
Infectious proteins with no DNA or RNA
Rare, but cause incurable brain infection
Cannot easily removed by sterilisation or disinfection
What are viruses?
RNA or DNA
Protein coat +/- lipid envelope
Obligate intracellular parasite
What are the diagnostic methods for parasites?
Viral cell culture
Antigen or antibody detection in blood (serology)
PCR to detect DNA or RNA
What are bacteria?
Contain DNA and RNA One double strand chromosome, sometimes with extra plasmids Cell membrane and cell wall +/- capsule Flagellae for movement Pili/fimbriae for adhesion
What are the bacterial shapes?
coccus = round bacillus = rod shaped spirochaetes = spirals
What colour does gram positive stain?
Purple
What colour does gram negative stain?
Pink
What does a gram positive cell wall look like?
Thick layer of peptidoglycan
This will hold the purple colour
What does a gram negative cell wall look like?
Thin layer of peptidoglycan
Cannot hold the purple colour, so shows pink
Why does mycobacterium tuberculosis not stain well with gram stain?
Has a thick waxy coat which does not take up the stain
What is a pathogen?
A harmful organism
What is a commensal?
An organism that is part of the normal flora
What is an opportunistic pathogen?
An organism that will probably only cause infection in n immunocompromised individual
What is a contaminant?
An organism that has got into the culture by a accident
What is pathogenicity?
The ability of a microorganism to produce disease
What is virulence?
The degree of pathogenicity of an organism (how easily the organism can cause disease)
By which method to bacteria replicate?
Binary fission
how quickly can bacteria double their numbers in ideal growth conditions?
Every 20 minutes
What are the four stages of the bacterial growth curve?
Lag phase = DNA replication
Log phase = Exponential growth
Stationary phase = Rate of growth equal to rate of death
Death phase = Limited nutrients, so bacteria compete and some die
What is an exotoxin?
Mostly produced by gram positive bacteria
Usually produced inside the cell then exported from it
What is an endotoxin?
Mostly produced by gram negative bacteria
Part of the gram negative bacterial cell wall, released when bacterial cell dies
What are the diagnostic methods for bacteria?
Microscopy (gram stained film) Culture Detection of antigen Detection of antibodies in blood (serology) PCR
What types of fungi are there?
Moulds = Produce spores and hyphae Yeasts = Reproduce through budding
What is the most common cause of mould infection in humans?
Aspergillus spp
usually in immunocompromised
What is the most common cause of fungal infection in humans?
Candida spp
Yeast
Stain as lage gram positive oval structures
What are the gram positive aerobic cocci?
Streptococci = chains Staphylococci = clusters
How do you differentiate between streptococcus species?
Haemolysis seen on blood agar
What are the possible results of streptococcal haemolysis?
Alpha haemolysis = partial haemolysis, greenish discolouration around colony
Beta haemolysis = complete haemolysis, clearing around colony, most virulent
Gamma haemolysis = no haemolysis
What are the alpha haemolytic streptococci?
Streptococcus pneumoniae
Streptococcus “viridans”
What infections can be caused by Streptococcus pneumoniae?
Commonest cause of pneumonia
Also causes meningitis
Where would the Streptococcus “viridans” normally colonise and what infection can they cause if they reach a normally sterile site?
Common commensals of the URT, bowel and vagina
Endocarditis
What are the beta haemolytic streptococci?
Group A strep (Strep. pyogenes)
Group B strep
What infections can be caused by Group A Streptococcus (Strep. pyogenes)?
Streptococcal sore throat (tonsillitis) (+ rash gives scarlet fever)
Skin and soft tissue infections (e.g. necrotising fasciitis)
Puerperal sepsis (life threatening infection in pregnant and recently post natal women)
What are the non-haemolytic streptococci?
Most important are the enterococci (part of normal bowel flora)
What infection can be caused by the Enterococci?
Commonest cause of UTI
Which test is used to distinguish between Staphylococcus species?
Coagulase test
What are the possible results if a Staphylococcus coagulase test?
Coagulase positive
Coagulase negative
What is the one coagulase positive Staphylococcus?
Staphylococcus aureus
Which infections can be caused by Staphylococcus aureus?
Commonest cause of skin, soft tissue and wound infection
Commonest cause of bone and joint infection
Enterotoxin producing strains cause food poisoning
Staph. aureus bacteraemia
What are the coagulase negative Staphylococci?
Many species of skin commensals
Inc. Staph epidermidis
What infections can be caused by coagulase negative Staphylococci?
Prosthetic joint and heart valve infection
IV catheter infection
Which antibiotics can be used to treat Streptococcus pneumoniae?
Most UK strains still resistant to penicillin
Which antibiotics can be used to treat Group A Streptococci?
All strains still sensitive to penicillin and amoxicillin
Which antibiotics can be used to treat Enterococci?
Most trains still sensitive to amoxicillin (not penicillin)
What is the antibiotic of choice for Staphylococcus aureus?
Flucloxacillin
What are antibiotic resistant strains of Staphylococcus aureus resistant to?
MRSA
All penicillins, all cephalosporins, plus others
What is pyrexia?
Raised temperature (fever)
What is rigor?
Shivering attack as raised temperature drops
Sign of fever or pyrexia
Above which temperature is fever diagnosed?
> 38 degrees C
What causes sepsis?
Small blood vessels become leaky in severe infection and fluid is lost into the tissues
Lower blood volume leads to septic shock
What are the gram negative aerobic cocci?
Diplococci (appear in pairs)
Neisseria meningitidis
Neisseria gonorrhoeae
What infection can be caused by Neisseria meningitidis?
Commonest cause of bacterial meningitis
What infection can be caused by Neisseria gonorrhoeae?
Gonorrhoea
What are the gram negative aerobic large bacilli?
Coliforms
Look like E.coli on gram film and blood culture
How are the coliforms differentiated from each other?
Biochemical reactions
Antigenic structure of cell wall (serotyping)
What are the gut commensal coliforms?
Most E.coli spp. Klebsiella spp. Enterobacter spp. Proteus spp. Many more
What are the gut pathogen coliforms?
Salmonella spp.
Shigella spp.
Verotoxin producing E.coli (E.coli O157, E.coli O104)
What infections can be caused by the coliforms?
Any coliform in a normally sterile site, e.g.
UTI
Peritonitis
Biliary tract infection
What is the first line antibiotic treatment for coliform infections?
Gentamicin
What are the gram negative aerobic small bacilli?
Pseudomonas aeruginosa Legionella pneumophila (Legionnaires' disease)
What are the gram negative microaerophilic bacilli?
Spiral or curved Gram negative bacilli =
Vibrio cholerae
Campylobacter spp
Helicobacter pylori
What infection is caused by Vibrio cholerae?
Cholera
What infection is caused by Campylobacter?
Commonest cause of bacterial diarrhoea in the UK
What infections can be caused by Helicobacter pylori?
Gastritis
Duodenal ulcers
What is the gram negative aerobic small (cocco)bacillus and what infection does it cause?
Haemophilus influenzae
Common cause of chest infection
What are the strict anaerobes?
Clostridium spp.
Bacteroides spp.
What is the classification of the Clostridium spp?
Gram postive anaerobic bacilli
What type of Clostridium spp. are there and what infections do they cause?
Clostridium difficile = Antibiotic associated diarrhoea
Clostridium perfringens = “Gas” gangrene
Clostridium tetani = Tetanus
What is the classification of the Bacteroides spp?
Gram negative anaerobic bacilli
What infections can be caused by the Bacteroides spp?
Usually non pathogenic
Only cause infection in sterile sites (e.g. peritoneum, biliary tract)
What is the first line treatment for infections caused by anaerobes?
Metronidazole
What are the Mycobacteria?
Have a thick waxy outer coat which does not take up Gram Stain
Stained by acid fast bacilli or acid-alcohol fast bacilli
What infection can be caused by Mycobacterium tuberculosis?
TB
What are the Spirochaetes?
Spiral shaped spectre that do not stain on gram stain
Treponema pallidum
Borrelia burgdorferi
What infection is caused by Treponema pallidum ?
Syphilis
What infection is caused by Borrelia burgdorferi?
Lyme disease
How does genetic variation in bacteria occur?
Mutation
Gene transfer
How do mutations in bacterial DNA cause genetic variation?
Rapid bacterial grwoth increases chance of genetic mutation
This leads to new variants of the organism
Many fail to survive, but rare strains can be successful
What are the three methods of gene transfer?
Transformation
Conjugation
Transduction
How does transformation lead to genetic variation?
DNA released from dead bacteria can be taken up by living bacteria and incorporated into palms or bacterial chromosome
How does conjugation lead to genetic variation?
Bacterial “sex”
Sex pilus is produced by one bacteria through which plasmids DNA can be transferred
How does transduction lead to genetic variation?
Viruses that infect bacteria can transfer bits of DNA from one bacteria to another
What are the main mechanisms of antibiotic resistance?
Production of enzymes that destroy antibiotics
Altered antibiotic binding sites
Alteration of cel wall porins
Up-regulation of efflux pumps
Which enzymes can be produced to destroy antibiotics?
Beta-lactamase = resistance to early penicillins like amoxicillin
Extended spectrum beta-lactamse = resistance to all penicillins and cephalosporins
Carbapenemase = resistance to all penicillins, all cephalosporins and carbapenems
How do altered antibiotic binding sites cause antibiotic resistance?
Genetic mutation alters antibiotic binding sites
Antibiotics that cannot bind have no effect on bacteria
Give an example of altered antibiotic sites leading to antibiotic resistance?
Alteration of penicillin binding protein on Staph aureus resulted in fluxloxacillin resistance (MRSA)
How do changes to cell wall porins cause antibiotic resistance?
Alterations to porins may result in bacterial cell wall becoming impermeable to the antibiotic
How does up regulation of efflux pumps cause antibiotic resistance?
Antibiotic is removed from the cell before it has time to act
Give an example of up regulation of efflux pumps leading to antibiotic resistance?
Common mechanism of resistance in fungi e.g. Candida spp.
Why must antibiotics be selectively toxic?
To kill bacteria without killing surrounding tissue
What is the difference between bactericidal and bacteriostatic?
Bactericidal = Kill bacteria Bacteriostatic = Inhibit bacteria growth
By which three mechanisms can antibiotics work?
Acting on bacterial cell wall = Human cells do not have a cell wall
Affecting the bacterial ribosome = Structurally different from human ribosomes
Acting in bacterial DNA =
Structurally different from human DNA
Which groups of antibiotics work on the bacterial cell wall?
Penicillins
Cephalosporins
Glyopeptides
What are the advantages of the penicillins?
Safe, very few side effects
Narrow to broad spectrum
Excreted rapidly via kidneys
Safe in pregnancy
What are the disadvantages of the penicillins?
Some patients are allergic - allergy to one means allergy to all of the family, and sometimes also cephalosporins
Frequent dosing needed due to rapid excretion
What are the three forms of penicillin available?
Benzylpenicillin (IV) Phenoxymethyl penicillin (Oral) Benzathine penicillin (IM)
Which penicillin can only be used for gram positive organisms and why might it be prescribed?
Flucloxacillin (IV, Oral)
Only used for Staphylococci and Streptococci
Skin and soft tissue infection
Wound infection
Cellulitis
Which penicillins can be used for gram positive and negative organisms?
Amoxicillin (IV, Oral)
Co-Amoxiclav (IV, Ora)
Tazocin (IV)
Why is Co-Amoxiclav more effective than Amoxicillin?
Clavunic acid is a beta-lactamase inhibitor, and so prevents the destruction of amoxicillin
Which penicillin can only be used for gram negative organisms and why might it be prescribed?
Temocillin (IV)
Only active against coliforms
Brought back due to increasing bacterial resistance
Still active against EBSL producing organisms
What are the advantages of the cephalosporins?
Excreted via kidneys and urine
Few side effects
Safe in pregnancy
What are the disadvantages of the cephalosporins?
Broad spectrum antibiotics, significantly affect the normal bowel flora
Increased risk of C.diff
Give examples of the glycopeptides?
Vancomycin (IV)
Teicoplanin (IV)
Excreted via kidneys and urine
What kinds of bacteria are the glycopeptides active against?
ONLY active against organisms with a gram positive cell wall
Are penicillins bactericidal or bacteriostatic?
Bactericidal
Are cephalosporins bactericidal or bacteriostatic?
Bactericidal
Are glycolpeptides bactericidal or bacteriostatic?
Bactericidal
Which groups of antibiotics inhibit protein synthesis?
The macrolides
The tetracyclines
The aminoglycosides
Others (clindamycin, chloramphenicol)
Are macrolides bactericidal or bacteriostatic?
Bacteriostatic
Are tetracyclines bactericidal or bacteriostatic?
Bacteriostatic
Are aminoglycosides bactericidal or bacteriostatic?
Bactericidal
Give examples form the macrolide family?
Erythromycin
Clarythromycin
Azithromycin
How are the macrolides excreted?
Via the liver, biliary tract and into the gut
Which macrolide is safe in pregnancy?
Erythromycin
Which kinds of infections are macrolides useful in treating?
Infections in which bacteria hide from hot cell immunity by getting into the host cells
Macrolides are lipophilic
Give an example of an aminoglycoside?
Gemtamicin (IV)
Which organisms is gentamicin effective against?
Gram negative aerobes such as coliform and pseudomonas aeruginosa
How is gentamicin excreted?
Via urine
In what way is gentamicin toxic?
Damages kidneys and CN VIII, causing dizziness and deafness
Must monitor bloods carefully
Which groups of antibiotics act on bacterial DNA?
Metronidazole
Trimethoprim (+/- sulphonamide)
Fluroquinolones
Which infections are metronidazole used to treat?
True anaerobic infections
Some protozoal infections
How can trimethoprim be administered?
On its own, orally
In combination with sulphamethoxazole, orally or IV, as co-trimoxazole (synergistic effect)
Which bacteria is trimethoprim effective against?
Some Gram negative and some Gram positive bacteria
How is trimethoprim excreted?
Via urine
Give examples of Fluroquinolones?
Ciprofloxaxin (IV, Oral)
Levofloxacin (IV)
Are fluroquinolones bactericidal or bacteriostatic?
Bactericidal
How are fluroquinolones excreted?
Via urine
Which antibiotic should you really not drink alcohol when taking?
Metronidazole
What are the 4 C antibiotics?
Cephalosporins
Co-Amoxiclav
Ciprofloxacin
Clindamycin
Why do we avoid use of the 4 C antibiotics?
Broad spectrum
Associated with increased risk of C.diff
