The collapsed traveller

Question 1

DESCRIBING BACTERIA - STAINING

i) what is the fundamental difference between gram positive and negative bacteria? which one contains LPS?
ii) what colour does gram pos stain? why? which colour does gram neg stain? why?
iii) what is the name for chains of cocci, clusters of cocci, rods?
iv) which bacteria grows in pairs?

Answer 1

i) differences in cell wall
- gram positive has thick peptidoglycan layer and gram negative is thin
- gram neg has LPS

ii) gram positive > purple due to lots of PG in cell wall
gram neg > pink due to less PG in cell wall > lose stain so counter stain punk

iii) chains of cocci = strep
clusters of cocci = staph
rods = bacilli

iv) strep pneum grows in pairs (pneumococcus)

Question 2

GRAM STAINING

i) label A-F - describe and which gram stain?

Answer 2

A- staph cocci (gram positive)

B - strep cocci (gram positive)

C - strep cocci (cocci growing in pairs = strep pneum) - gram positive

D - bacilli (gram positive)

E - e coli - bacilli (gram negative)

F - cocci (gram negative) -neissieria meningitis

Question 3

BACTERIAL GROWTH

i) what defines an aerobe? give an example?
ii) what is an obligate anerobe? give an exmaple
iii) what is a faculative anerobe? give an example
iv) what is alpha haemolysis? what colour is produced?
v) what is beta haemolysis? how does the agar appear
vi) what is gamma haemolysis? which type of bacteria is haemolysis used in the classification of?

Answer 3

i) aerobe > use oxygen as final electron acceptor eg staph aureus
ii) obligate anaerobe > only metabolise anaerobically eg clostridioides spp
iii) facultative anerobe > switch between aerobe and anaerobe eg ecoli
iv) alpha haem > partial haemolysis of blood agar > prod green colour
v) beta haem > complete haemol of agar > translucent

vi) gamma haemol = no haemolysis
- used for streptococci

Question 4

STAPH AUREUS

i) name an area where it is commensal? name two features of infection?
ii) how does it gram stain? how does it appear on micro?
iii) name three things that contibute to staph aureus virulence? what are the two phases and which phase are toxins secreted?

Answer 4

i) commensal of the nose
- infec > abscess, impetigo, furunculosis (swollen nose)

ii) gram positive cocci in clusters

iii) coagulase enzyme, thin polysacch cap to evade imm sys, exotoxins, adhesins allow sticking to host
- exponential phase = surface proteins
- stationary phase = secreted toxins

Question 5

COAGULASE NEGATIVE STAPHLOCOCCI

i) how do they appear on staining?
ii) what do they often act as? in which patients may they cause disease?
iii) are they more or less virulent than coag positive strains?
iv) name the most common species? where does it live and what does it commonly contaminate?

Answer 5

i) gram positive cocci in clusters

ii) often act as contaminants
- can cause disease in patients with implanted metal devices

iii) less virulent > differen in the lab with the coagulase test

iv) mst common is staph epidermidis > lives on the skin and commonly contaminates blood cultures
- can also contaminate grafts, implants, prosthetic valves

Question 6

CELLULITIS

i) what is the most common causative organism? what is seen on gram stain? what type of haemolysis does it do?
ii) name another bacteria that less commonly causes it?
iii) are blood cultures always positive?
iv) how does it present?

Answer 6

i) strep pyogenes > gram positive cocci growing in chains
- beta haemolysis of agar

ii) staph aureus
iii) blood cultures are sometimes positive
iv) presents with hot, red, inflamed leg

Question 7

STREPTOCOCCI

i) what are the three ways to classify it?
ii) what colour results from alpha haemolysis? name two types of strep that act in this way? where are they usually commensals?

Answer 7

i) 1) appearance on blood agar (haemolysis)
2) lancefield groups (A,B.C) > surface carbohydrate antigens eg group A strep
3) true species name eg strep pyog, strep pneum

ii) green (viridans) > s milleri, s pneumonia
- commensals in the mouth

Question 8

NECROTISING FASCITIS

i) what is it clinically? what is it hard to distinguish from initially?
ii) which group of people does it most commonly occur in? what is the most common causative organism?
iii) what causes it? what type of pain is felt?
iv) name three symptoms? which organ may be affected? which blood marker will be very high?
v) what is the only way to diagnose it?

Answer 8

i) clinically it is bad cellulitis with signs of sepsis
- hard to disting from simple cellulitis initially

ii) most common occurs in young healthy people
- strep pyogenes infection > deep infec that tracks the fascial planes

iii) caused by the production of tissue destructive enzymes by organsism in stationary phase
- pain out of proportion to clinical signs

iv) bruising, blistering, general toxaemia (flushed, fever, confued)
- very high CRP
- CK may be raise
- renal impairment

v) only way to dx is by surgical exploration

Question 9

STREP PYOGENES

i) name two superficial infections that it causes
ii) name three deep infections it causes
iii) name two auto immune sequale? are these common or rare in the UK?

Answer 9

i) superficial - pharyngitis and cellulitis
ii) deep - severe soft tissue infection, myositis, nec fasc

iii) rheum fever and glomnephritis
- rare in UK

Question 10

MENINGOCOCCAL SEPSIS

i) what pathogen causes it? which two areas does the bacteria adhere to?
ii) what is a key component of the bacterial cell wall? what type of gram stain bacteria causes it?
iii) what is happening when blebbing is seen on microscopy?

Answer 10

i) neiserria meningitidis
- adheres to resp epithelium and meninges

ii) LPS in cell wall > gram negative (stain with indian ink)

iii) blebbing - when n.meningitidis sheds its LPS
- LPS stimulates a powerful innate immune response responsible for many disease manifestations

Question 11

E COLI

i) where are these bacteria commensal? is it gram positive or negative?
ii) are they enterococcus? what type of sepsis is it the most common cause of?
iii) when do opportunisitc infections occur? name two instances?
iv) which three major human diseases does E coli cause?
v) by which four mechanisms does it confer virulence?

Answer 11

i) in the gut
- gram negative

ii) not enterococcus
- most common cause of urosepsis

iii) opp infections when organisms access sterile sites eg wound infections, biliary infections, HAP
iv) UTI, enteric, meningitis in neonates, elderly, immunocomp
v) adhesins, siderphores (iron chelator essential for survival), capsule (protects from complement responses) and toxins (LPS)