Flashcards in Microbiology and Infectious Disease Deck (195)
an organism that is capable of causing disease
organism which colonises the host but does not cause disease in normal circumstances
define opportunist pathogen
microbe that only causes disease if host defences are compromised
the degree to which a given organism is pathogenic
define asymptomatic carriage
when a pathogen is carried harmlessly at a tissue site where it causes no disease
describe the structure of a Gram negative bacterial cell envelope
inner cell membrane,
thin peptidoglycan layer,
outer cell membrane,
describe the structure of a Gram positive bacterial cell envelope
thick layer of peptidoglycan between the outer capsule + inner cell membrane.
what is a bacterial endotoxin? describe features
a component of the cell wall that is released when the bacteria is damaged.
less specific actions than an exotoxin e.g. septic shock
what is a bacterial exotoxin? describe features
mainly excreted by Gram +ve bacteria. actively secreted toxins with specific actions.
how might bacterial genes be transferred between bacteria?
transformation e.g. via plasmid.
transduction e.g. via phage.
conjugation e.g. via sex pilus.
describe the process of Gram staining
1. heat fix sample to slide
2. add methyl violet (blue/purple)
3. add iodine - fixes methyl violet to gram +ve samples
4. add alcohol to decolorize Gram -ve samples
5. counterstain with basic fuchsin (red)
what is the normal habitat of staphylococcus spp?
nose and skin
how is S aureus spread?
aerosol and touch
what agar is used to grow enterobacteria?
MacConkey - bile salts, lactose, pH indicator
How do you distinguish Salmonella/Shigella from E coli on a MacConkey-lactose agar?
E.coli = pink, as it is lactose fermenting
Shigella/salmonella = yellow, non-lactose fermenting
how would you distinguish between Salmonella and Shigella?
serology, as both are non-lactose fermenting so appear the same on MacConkey agar
what are the main infections caused by pathogenic E coli strains?
wound infections (surgical).
what 3 infections are caused by salmonella?
1. gastroenteritis - food poisoning (localised infection)
2. enteric fever - typhoid (systemic infection)
3. bacteraemia - uncommon
what agar must H influenzae be grown on and why?
fastidious - requires haem and NAD - will not grow on blood agar, only chocolate agar (blood agar that has been heated so haem and NAD are released by RBCs).
describe the main features of Legionella pneumophila and Legionnaires' disease
seen in immunocompromised (elderly, alcoholics, smokers).
culture on charcoal agar.
found in man-made aquatic environments - replicates within freshwater protozoa.
can infect alveolar macrophages.
what are the clinical features of an infection with campylobacter?
mild to severe diarrhoea, often with blood.
self-limiting (up to 1 wk).
campylobacter shed in faeces for 3wks.
describe the features of bacteroides? where are they typically found as commensals?
commensal flora of large intestine (also commensal in vagina/cervix).
what shape are bacteroides?
what shape are spirochaetes?
describe the cell wall of fungi and how it stains?
rigid. polysaccharides and chitin.
stain with Gomorra methenamine silver, and periodic acid-Schiff.
lack a capsule.
describe the features of yeasts
asexual - reproduce by budding.
what are dimorphic fungi?
fungi which grow as yeasts in tissue, but as moulds in-vitro.
describe the features of moulds
composed of tubular structures (hyphae).
grow by longitudinal extension and branching - interwoven mycelium.
reproduce by spore formation (sexual/asexual).
what type of fungi is candida albicans?