Microbiology and Infectious Disease Flashcards Preview

Phase 2a - Introductory Clinical Sciences > Microbiology and Infectious Disease > Flashcards

Flashcards in Microbiology and Infectious Disease Deck (195)
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1

define pathogen

an organism that is capable of causing disease

2

define commensal

organism which colonises the host but does not cause disease in normal circumstances

3

define opportunist pathogen

microbe that only causes disease if host defences are compromised

4

define virulence/pathogenicity

the degree to which a given organism is pathogenic

5

define asymptomatic carriage

when a pathogen is carried harmlessly at a tissue site where it causes no disease

6

describe the structure of a Gram negative bacterial cell envelope

inner cell membrane,
thin peptidoglycan layer,
outer cell membrane,
lipopolysaccharide layer,
outer capsule.

7

describe the structure of a Gram positive bacterial cell envelope

thick layer of peptidoglycan between the outer capsule + inner cell membrane.

8

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

9

what is a bacterial exotoxin? describe features

mainly excreted by Gram +ve bacteria. actively secreted toxins with specific actions.

10

how might bacterial genes be transferred between bacteria?

transformation e.g. via plasmid.
transduction e.g. via phage.
conjugation e.g. via sex pilus.

11

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)

12

what is the normal habitat of staphylococcus spp?

nose and skin

13

how is S aureus spread?

aerosol and touch

14

what agar is used to grow enterobacteria?

MacConkey - bile salts, lactose, pH indicator

15

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

16

how would you distinguish between Salmonella and Shigella?

serology, as both are non-lactose fermenting so appear the same on MacConkey agar

17

what are the main infections caused by pathogenic E coli strains?

wound infections (surgical).
UTIs.
gastroenteritis.
travellers' diarrhoea.
bacteraemia.
meningitis.

18

what 3 infections are caused by salmonella?

1. gastroenteritis - food poisoning (localised infection)
2. enteric fever - typhoid (systemic infection)
3. bacteraemia - uncommon

19

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).
non-motile.

20

describe the main features of Legionella pneumophila and Legionnaires' disease

seen in immunocompromised (elderly, alcoholics, smokers).
severe.
culture on charcoal agar.
found in man-made aquatic environments - replicates within freshwater protozoa.
can infect alveolar macrophages.
induces phagocytosis.

21

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.

22

describe the features of bacteroides? where are they typically found as commensals?

non-motile rods.
strict anaerobes.
commensal flora of large intestine (also commensal in vagina/cervix).

23

what shape are bacteroides?

bacilli

24

what shape are spirochaetes?

spiral/helical

25

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.

26

describe the features of yeasts

unicellular.
round/oval.
asexual - reproduce by budding.

27

what are dimorphic fungi?

fungi which grow as yeasts in tissue, but as moulds in-vitro.

28

describe the features of moulds

composed of tubular structures (hyphae).
grow by longitudinal extension and branching - interwoven mycelium.
reproduce by spore formation (sexual/asexual).

29

what type of fungi is candida albicans?

yeast

30

give some examples of moulds

aspergillus spp.,
fusarium spp.
mucoraceous moulds.
dark-walled fungi.