classification of pathogenic bacteria Flashcards
describe the classification of gram-positive bacteria:
Aerobic & Anaerobic
Morphology:
Cocci (round) → Chains & Clusters
&
Bacilli (rods) → Small & Large
Chains → Streptococci → Haemolysis classification
Haemolysis: appearance on blood agar
- alpha - partial lysis
- beta - complete lysis
- non - no haemolysis
Clusters → Staphylococci → Coagulase ( + / - ) classification
deescribe staph.aureus:
coagulase-positive staphylococci i.e. produces the enzyme coagulase.
commensal organism - nose, axilla, perineum.
MRSA → methicilin-resistant Staph. aureus
MSSA → methicilin sensitive Staph. aureus
PRSA → penicillin-resistant Staph. aureus
(production of penicillinase
Wide range of diseases from boils/abscesses and soft tissue infections to septicaemia and osteomyelitis.
what are the major genus of aerobic gram-positive cocci?
Staphylococci (clusters) and Streptococci & Enterococci (chains)
One of the common characteristics of coagulase-negative staphylococci is the formation of biofilms…
i.e. when they adhere to a surface and form a community of bacteria, which is very difficult to treat.
Many different species e.g. S. epidermidis, S. haemolyticus, S. saprophyticus, S. lugdunensis.
There may be significant pathogens in the presence of foreign bodies/prostheses or immunocompromised individuals.
describe what s.lugdunensis and s.epidermidis is:
S. lugdunensis - a sneaky coagulase-negative staphylococci that behaves like S. aureus in regards to pathogenesis.
S. epidermidis - a coagulase-negative staphylococci that are mainly skin commensals which can be pathogenic in the presence of foreign bodies/prostheses or immunocompromised individuals
what are the haemolytic classifications of streptococci?
a-haemolytic (partial haemolysis)
→ turn blood agar green
b-haemolytic (complete haemolysis)
→ turn blood agar clear
Non-haemolytic
→ no difference to blood agar
what are the two important species of a-haemolytic streptococci?
1- Streptococcus pneumoniae (pneumococcus)
→ pneumonia, meningitis, septicaemia
2- Viridans Streptococci
→ Many different species
→ Normal oral flora
→ Cause of infective endocarditis (infection of heart valves)
b-haemolytic are further idenitifed by carbohydrate surface antigens, lancefield group A-G, describe groups a,b and d:
[1] Group A streptococci
* Streptococcus pyogenes
- Major pathogen
- Pharyngitis, cellulitis, necrotising fasciitis (“flesh eating bug”)
[2] Group B streptococci
* Streptococcus agalactiae
- Neonatal sepsis
- vertical transmission: mother to child during pregnancy
– Meningitis
– Bacteraemia
- Genital tract carriage common as a commensal (around 25% women).
- Also responsible for invasive infections in adults
(often older and underlying factors e.g. diabetes, liver disease / alcohol abuse, CVD, malignancy).
[3] Group D streptococci
* 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 (UTI) and infective endocarditis.
what are the 3 gram positive bacilli (Aerobic & Anaerboic)?
[1] Clostridiodes/Clostridium difficile
- “Difficult” to culture.
- Asymptomatic gut carriage –up to 16% of adults and 66% of babies.
- Increased risk with antibiotic use → disrupts the normal gut flora.
- causes diarrhoea, associated with toxin production and potentially fatal.
- Pseudomembranous colitis.
- Spread / transmitted via spores.
(bacteria hibernation - alcohol doesn’t kill it) - Detect antigen & toxin in stool sample by ELISA.
[2] Clostridium perfringens
- Found in soil and normal commensal in human and animal gut/faeces.
- Can contaminate food and cause gastroenteritis (enterotoxin-producing strains).
- Infects wounds may cause “gas gangrene”.
[3] Clostridium tetani - less important/common (preventable by vaccine)
- Toxin product ion by C. tetanicause tetanus.
- Uncontrolled muscle spasm due to loss of inhibition at neuromuscular junction.
- Antigenically modified toxin (toxoid) used for immunisation.
what are the terms infection and colonisation?
Infection - germs in or on the body and make you sick
Colonization - germs are on the body but do not make you sick
75% of the worst bacteria as prioritized by WHO are Gram negative bacteria
describe (Neisseria spp) & (Moraxella spp):
[1] e.g. Neisseria meningitidis (meningococcus)
- Meningitis – inflammation of the meninges and septicaemia (blood poisoning).
- Diagnosed by taking blood cultures or CSF.
- Often life threatening (some vaccines are made for some strains)
[2] e.g. Neisseria gonorrhoeae (less relevant)
- Causes urethritis in men and pelvic inflammatory disease in women.
- Spread by sexual contact.
[3] Moraxella catarrhalis (less relevant)
- Causative agent of respiratory tract infections.
- Especially in those with underlying lung pathology.
what is septicaemia and sepsis?
Septicaemia - when bacteria enter the bloodstream, and cause blood poisoning which triggers sepsis.
Sepsis - an overwhelming and life-threatening response to infection that can lead to tissue damage, organ failure and death.
what are coliform?
gram negative bacilli organisms
Enterobacteriaceae family
mainly commensals of the human large intestine.
what test is useful in the preliminary test in classifying gram negative bacilli?
Lactose fermentation
