Classification of pathogenic bacteria-gram positive

Question 1

Differences between gram positive cell wall and gram negative?

Answer 1

Gram negative:
more complex cell wall
same cytoplasmic membrane
periplasmic space in between the peptidoglycan thin layer and cytoplasmic membrane
outer membrane (contains lipopolysaccharide which is responsible for endotoxic shock in gram negative)

Question 2

What is peptidoglycan?

Answer 2

sugar network
composed of two major sugars
n-acetylmuramic acid and n-acetylglucosamine

Question 3

What do purple and pink stains from gram stain show?

Answer 3

purple bacteria- gram positive
pink bacteria-gram negative

Question 4

How would a gram stain be conducted?

Answer 4

heat fix bacteria to slide and flood with crystal violet
then add iodine
decolourisation step using alcohol
use counter stain using alcohol

Question 5

Why is gram positive purple?

Answer 5

thick layer of peptidoglycan can form complexes within the peptidoglycan and stain retained

Question 6

What forms do gram positive aerobic cocci take?

Answer 6

chains- streptococci
clusters-staphylococci

Question 7

How are streptococci classified?

Answer 7

according to their hemolysis- appearance on blood agar
betahemolytic- complete detsruction of surrounding red blood cells
alphameolytic- partial
non-hemolytic- no effect

Question 8

How can staphylococci be classified?

Answer 8

on whether can produce enzyme coagulase.
Coagulase positive- pathogens
coagulase negative- less pathogenic

Question 9

How do we classify gram positive bacilli?

Answer 9

size of bacillus

large - bacillus

small- corynebacetrium or listeria

Question 10

How do we classify anaerobes?

Answer 10

cocci or bacilli

Question 11

What is staphylococcus aureas?

Answer 11

• Commensal organism carried in nose, axilla, perineum. (can live harmlessly in these)
• A major human pathogen!
• Wide range of disease from boils/abscesses and soft tissue
  infections to septicaemia and osteomyelitis

Question 12

Antimicrobial patterns with staphylococcus aureas?

Answer 12

• Commonly penicillin resistant due to production of penicillinase.
• By a different mechanism, some strains are “methicillin
  resistant”.
• Methicillin resistant Staph aureus (MRSA) poses major problems
  for infection prevention and control in hospitals.

Question 13

What are some characteristics of coagulase negative staphylococci?

Answer 13

Many different species e.g. S. epidermidis, S. haemolyticus, S.
saprophyticus.
* Beware of S. lugdunensis.
* Mainly skin commensals.
* Form biofilms and may be significant pathogens in the
presence of foreign bodies/prostheses e.g. prosthetic heart
valves, prosthetic hip joints, pacemaker wires etc or
immunocompromised e.g. neonates / BMT with central
venous catheters

Question 14

How do you classify streptococci based of f of hemolysis?

Answer 14

alpha haemolytic streptococci- partial haemolysis and turns blood agar green

beta haemolytic- complete haemolysis and turns blood agar clear

non haemolytic- no difference

Question 15

What does streptococcus pneumoniae cause? (alpha haemolytic)

Answer 15

pneumonia, meningitis, septicaemia

has a capsule that helps protect it from immune system

Question 16

What do viridans streptococci cause? (alpha haemolytic streptococci)

Answer 16

typically colonise and reside in the oral cavity.

Question 17

How can beta haemolytic streptococci be further classified?

Answer 17

further identified by carbohydrate surface antigens (Lancefield group)

Groups A-G

A,B,F and D clinically most important

Question 18

Describe group A strep?

Answer 18

• Streptococcus pyogenes
• Major pathogen-Pharyngitis, cellulitis, necrotising fasciitis (“flesh
  eating bug”)

Question 19

Describe group B streptococci?

Answer 19

• Streptococcus agalactiae
• Neonatal sepsis– Meningitis– Bacteraemia
• Genital tract carriage common (around 25% women).
• Also responsible for invasive infections in adults (often older and
  underlying factors e.g. diabetes, liver disease / alcohol abuse, CVD,
  malignancy).

Question 20

Describe Group D streptococcus?

Answer 20

• Now re-classified as Enterococcus spp.
  E.g. Enterococcus faecalis, Enterococcus faecium
• Often non-haemolytic.
• Found in the gut as a normal commensal.
• Cause of urinary tract infection and infective endocarditis

Question 21

How do we classify gram positive bacilli?

Answer 21

aerobic

anaerobic

Question 22

Describe clostridiodes difficile? (gram positive bacilli)

Answer 22

• “Difficult” to culture.
• Asymptomatic gut carriage in healthy people – up to 16% of adults and
  66% of babies.
• Important cause of diarrhoea, associated with toxin production and
  potentially fatal.
• Increased risk with antibiotic use and anything else that disrupts the
  normal gut flora.
• Pseudomembranous colitis.
• Spread / transmitted via spores.
• Detect antigen & toxin in stool sample by ELISA

Question 23

Describe clostridium perfringens?

Answer 23

• Found in soil and normal commensal in human and animal
  gut/faeces.
• Can contaminate food and cause gastroenteritis (enterotoxin
  producing strains).
• Infects wounds may can cause “gas gangrene”

Question 24

Describe clostridium tetani?

Answer 24

Toxin production by C. tetani cause tetanus.

Uncontrolled muscle spasm due to loss of inhibition at neuro
muscular junction.

Antigenically modified toxin (toxoid) used for immunisation.

vaccine preventable disease

Question 25

What is the source of botox?

Answer 25

clostridium botulinum

Question 26

what is cause of anthrax?

Answer 26

bacillus anthracis