Microbiology Flashcards
What is a pathogen?
Organism that causes or is capable of causing disease
What is a commensal?
Organism which colonises host but causes no disease in normal circumstances
What is an opportunist pathogen?
Microbe that only causes disease if host defences are compromised
What is virulence?
Degree to which a given organism is pathogenic
What is asymptomatic carriage?
When a pathogen is carried harmlessly at a tissue site where it causes no disease
What are round bacteria called?
Cocci
What are rod shaped bacteria called?
Bacilli
What is a purple Gram stain?
Positive
What is a pink Gram stain?
Negative
What specialised stain detects mycobacterium??
Ziehl-Neelsen stain
What gives bacteria a hardened structure?
Spores
What component of a bacterial outer membrane can give rise to toxic shock?
Endotoxin
How big are typical bacterial chromosomes?
2-4x103 kb
How can bacteria be mutated?
Base substitution
Deletion
Insertion
How can genes be transferred in bacteria?
Transformation
Transduction
Conjugation
What is coagulase?
An enzyme produced by bacteria that clots blood plasma to protect against phagocytosis
What is an important coagulase positive bacteria?
Staphylococcus aureus
What is the normal habitat for staphylococcus?
Nose and skin
How is staph aureus spread?
Aerosol and touch
What is MRSA resistant to?
B-lactams, gentamicin, erythromycin, tetracycline
What are the virulence factors of Staph aureus?
- Pore-forming toxins
- Proteases (exfoliatin)
- Toxic shock syndrome toxin
- Protein A
Give 3 pyogenic conditions associated with Staph aureus
- Wound infections
- Septicaemia
- Abscesses
Give 2 toxin mediated conditions associated with staph aureus
- Toxic shock syndrome
2. Food poisoning
Give 2 coagulase negative conditions associated with Staph aureus
- Infected implants
2. Endocarditis
What is the main virulence factor for S. epidermidis?
Ability to form persistent biofilms
What are the 3 types of haemolysis?
- Alpha - partial lysis (greening)
- Beta - complete lysis
- Gamma (non) - no lysis
How can streptococci be classified?
- Haemolysis
- Lancefield typing
- Biochemical properties
What is Lancefield grouping?
Method of grouping catalase negative, coagulase negative bacteria based on bacterial carbohydrate cell surface antigens
Give 3 infections caused by S. pyogenes
- Cellulitis
- Tonsillitis
- Scarlet fever
What are the 2 complications of S. pyogenes infections?
- Rheumatic fever
2. Glomerulonephritis
What are the virulence factors for S. pyogenes?
- Enzymes
- Toxins
- Capsule
- M protein
What are the causes of S. pneumoniae?
- Pneumonia
- Otitis media
- Sinusitis
- Meningitis
What are the predisposing factors for S. pneumoniae infection?
- Impaired mucus trapping
- Hypogammaglobulinaemia
- Asplenia
Which streptococci are important in infective endocarditis?
- S. sanguinis
2. S. oralis
What is the most virulent group of streptococci?
Milleri group
Why are there differences in staining with Gram?
Morphological differences in the bacteria
What is endotoxin comprised of?
Lipid A, core antigen, somatic antigen
How does lipid A trigger an innate immune response?
TLR4 signalling
What are the main colonisation factors?
- Adhesins
- Invasins
- Nutrient acquisition
- Defence against host
Why might antibodies against the same species of bacteria not be recognised in different patients?
There is structural variability amongst different strains of the same species
What is a serovar?
Strains of bacteria with variant cell surface antigens
What is a serogroup?
A group of strains with have a surface antigen in common, but differ in one or more other surface antigens
Name 6 principal infections caused by E. coli
- Wound infections
- UTIs
- Gastroenteritis
- Traveller’s diarrhoea
- Bacteraemia
- Meningitis
What is the major complication of E. coli food poisoning?
Haemolytic ureic syndrome
How many patients with a EHEC infection develop haemolytic ureic syndrome?
10%
What are the 5 symptoms of a shigella infection?
- Small volume
- Pus and blood in stools
- Prostrating cramps
- Pain in straining
- Fever
How is shigella spread?
Person-to-person, contaminated food and water
How do shigella invade epithelial cells?
Through M cells
How does Shigella ensure survival in macrophages?
It rapidly induces apoptosis of macrophages
What are the 2 species of salmonella?
- S. enterica
2. S. bongori
What are the 3 conditions caused by S. enterica?
- Gastroenteritis/enterocolitis
- Enteric fever
- Bacteraemia
What can klebsiella pneumoniae cause?
- UTI
- Pneumonia
- Surgical wound infections
- Bacteraemia
- Sepsis
How is cholera transmitted?
- Shellfish ingestion
2. Contaminated drinking water
How much fluid a day can be lost in cholera?
20L/day
What is the mortality for untreated cholera?
50-60%
What is the leading cause of nosocomial pneumonia in ICU patients?
Pseudomonas aeruginosa
Name 5 opportunistic infections caused by haemophilus influenzae
- Meningitis
- Bronchopneumonia
- Epiglottitis
- Bacteraemia
- Pneumonia
What is the main cause of meningitis
Type b strain of H. influenzae
What is the mortality of Legionnaires’ disease?
15-20%
What causes Legionnaires’ disease?
Legionella pneumophila
What does Legionnaires’ disease cause?
Excessive influx of neutrophils into lungs causing severe inflammatory pneumonia
What does bordetella pertussis cause?
Whooping cough
How is B. pertussis transmitted?
Aerosol
What are the 2 clinically important species of neisseria?
- N. meningitidis
2. N. gonorrhoeae
In what % of the population is N. meningitidis present in the nasopharynx?
5-10%
What is the mechanism of N. meningitidis?
Invasion of meninges, bacteria enter CSF of subarachnoid space from blood stream after crossing BBB
What serious complications can gonorrhoea cause in women?
Salpingitis
Pelvic inflammatory disease
What is the most common cause of food poisoning in UK?
Campylobacter
What diseases does Helicobacter pylori play a role in?
Gastritis
Peptic ulcer disease
What do bacteroides often cause?
Peritoneal cavity infections
How is chlamydia detected?
Serum antibodies or PCR
What are the 2 developmental stages of chlamydia?
- Elementary bodies
2. Reticulate bodies
Describe the elementary body stage of chlamydia development
- Infectious
- Enter cell through endocytosis
- Prevent phagosome-lysosome fusion
Describe the reticulate body stage of chlamydia development
- Replicative
- Non-infectious
- Acquire nutrients from host cell
What C. trachomatis causes chlamydia in STI form?
Genital tract biovar
What are the symptoms of Weil’s disease?
- Jaundice
- Acute renal failure
- Acute hepatic failure
- Pulmonary distress
- Haemorrhage
What causes Weil’s disease?
Leptospira interrogans
What is the primary stage of syphilis?
A localised infection, days-weeks post infection usually in the form of an ulcer
What is the secondary stage of syphilis?
A systemic infection, 1-3 months post-infection affecting skin, joints and muscles
What is the tertiary stage of syphilis?
Infection of bone and soft tissue, aorta and nervous system, several years post-infection
What are the 4 main groups of gram negative pathogens?
- Proteobacteria
- Bacteroides
- Chlamydia
- Spirochaetes
Give 3 characteristics of fungi
- Eukaryotic
- Chitinous cell wall
- Heterotrophic
What forms can fungi exist in?
Yeast or mould
What are yeasts?
Small single celled organisms that divide by budding
What are moulds?
Fungi which form multicellular hyphae and spores
What are dimorphic fungi?
Fungi which exist as both yeasts and moulds switching between the two when conditions suit
Why do fungi often not cause human infection?
- Inability to grow at 37C
2. Innate and adaptive immune response
What genera of fungi are most often implicated in human infection?
- Ascomycota
- Basidiomycota
- Mucormycota
Name 3 common fungal infections
- Vulvovaginal candidiasis
- Otitis externa
- Tinea pedis
What life-threatening fungal infections can affect immunocompromised hosts?
- Candida spp. line infections
- Invasive aspergillosis
- Pneumocystis
- Cryptococcosis
- Mucormycosis
What ways are there of diagnosing fungal disease?
- Microscopy and histology
- Culture
- Molecular methods and serology
Name 3 fungi in the DDx of onychomycosis
- Trichophyton spp.
- Epidemophyton spp.
- Microsporum spp.
Name 3 non-fungal causes considered in the DDx of onychomycosis
- Psoriasis
- Trauma
- Lichen planus
What is the diagnosis of onychomycosis?
Microscopy and culture of nail clippings/scrapings
What are the most common treatments for onychomycosis?
Terbinafine or itraconazole
Topical amorolfine
What is thrush?
White adherent plaques on oral or genital mucosa
Name 4 things commonly associated with thrush
- Immunocompromised
- Diabetes
- Antibiotic use
- Pregnancy
How is thrush treated?
Topical antifungals or oral fluconazole
What is invasive aspergillosis treated with?
Voriconazole
Who does invasive aspergillosis often affect?
Immunocompromised and post-influenza disease
What causes invasive aspergillosis?
Galactomannan
How is pneumocystis diagnosed?
PCR of induced sputum or BAL
How is pneumocystis pneumonia treated?
Co-trimoxazole
Why are fungi harder to treat than bacteria?
They are eukaryotic
What do fungal cell membranes contain?
Ergosterol
Where do allylamines tend to distribute extensively?
To poorly perfused sites e.g. skin and nail beds
What is used to treat dermatophytes?
Terbinafine
What are azoles?
Dose dependent inhibitors of 14a-sterol demethylase
Name 3 azoles
- Fluconazole
- Itraconazole
- Voriconazole
What is voriconazole used to treat?
Invasive aspergillosis
Give 3 side effects of azoles
- Transaminitis
- Alopecia
- Severe hepatitis
What class of drugs are used for severe or resistant fungal disease?
Echinocandins
Name 2 echinocandins
- Caspofungin
2. Micafungin
What is the mechanism of action of echinocandins?
Inhib 1,3 beta glucan synthase
What is the most common fungal cause of onychomycosis?
Trichophyton tubrum
Who does pneumocystis pathogenesis often affect?
Moderate-severely immunocompromised
Give 3 treatments of pneumocystis pathogenesis
- Co-trimoxazole
- Clindamycin
- Atovaquone
What is a virus?
An infectious, obligate intracellular parasite comprising genetic material surrounded by a protein coat and/or a membrane
What do viruses exist as when not in an infected cell?
Virions
Why are viruses not classed as living?
They don’t feed, respire or reproduce independently
How do viruses replicate?
- Migration of genome to host cell nucleus
- Transcription to mRNA using host materials
- Use cell materials for their replication
- Translation of viral mRNA to produce structural proteins, viral genome and non-structural proteins
How are new virus particles released?
- Cell lysis
- Budding
- Exocytosis
How can viruses cause disease?
- Direct destruction of host cells
- Modification of host cells
- Over-reactivity of immune system
- Damage through cell proliferation
- Evasion of host defences
How does poliovirus cause disease?
Host cell neuron lysis and death
How does rotavirus cause disease?
Atrophies villi and flattens epithelial cells
What are 4 symptoms of hepatitis B?
- Jaundice
- Pale stool
- Dark urine
- RUQ pain
How does human papillomavirus cause cervical cancer?
Continuous expression of oncoprotein causing cellular DNA mutations
How can the host defence be evaded at a cellular level?
- Latency
2. Cell-cell spread
How can the host defence be evaded at a molecular level?
- Antigenic variability
2. Prevention of host cell apoptosis
What is antigenic variability?
Ability to change surface antigens in order to evade host’s immune system
What does prevention of host cell apoptosis allow?
Allows virus to continue replicating within the cell so more virus is produced and then released
Name 2 ways viruses evade host defence
- Downregulation of interferon and other intracellular host defence proteins
- Interference with host cell antigen processing pathways
What are protozoa?
Microscopic unicellular eukaryotes
What are the major groups of protozoa?
- Flagellates
- Amoebae
- Microsporidia
- Sporozoan
- Ciliates
What is African trypanosomiasis also called?
Sleeping sickness
What is the vector for African trypanosomiasis?
Tsetse fly bite
Where does African trypanosomiasis occur?
Remote areas of Africa
What are the symptoms of African trypanosomiasis?
Personality change Drowsiness Flu-like symptoms Chancre Coma and death
How is African trypanosomiasis diagnosed?
Blood film or CSF
What is American trypanosomiasis also called?
Chagas disease
What is the vector for American trypanosomiasis?
Triatomine bug
Where is American trypanosomiasis found?
Central and South America
What are 3 acute symptoms of American trypanosomiasis?
- Fever
- Headache
- Romana’s sign
What are 3 chronic symptoms of American trypanosomiasis?
- Cardiomyopathy
- Megaoesophagus
- Megacolon
What causes Leishmaniasis?
Leishmania spp.
What is the vector for Leishmaniasis?
Female sandfly
Where is Leishmaniasis found?
Africa, Asia, South America
What are the 3 clinical manifestations of Leishmaniasis?
- Cutaneous
- Mucocutaneous
- Visceral Kala Azar