Staphylococci
Characteristics
- gram positive cocci
- arranged in irregular clusters / grapes
- non motile
- non spore forming
Culture
Grow on most media in aerobic conditions
- S aureus is golden
- S epidermidis is white
Resistance
ß lactamase production
- plasmid mediated
- resistant to some penicillins & cephalosporins
Methicillin Resistance
- independent of ß lactamase production
- mechanism unknown
- function of cell wall structure
Tolerance
- inhibition without death
- due to lack of activation of autolytic enzymes (PBP) in cell wall
Toxins & Enzymes
1. Exotoxin
- A-haemolysin
- ß-haemolysin
- endotoxin C
- haemolytic & pyogenic
2. Enterotoxin
- food poisoning
3. Coagulase
- clots citrated plasma
- produced by S. aureus
- S. epidermidis is coagulase negative
4. Other
- hyaluronidase
- staphylokinase
- exfoliative toxin (Toxic Shock Syndrome)
Pathogenesis
S. aureus
- pathogenic & invasive
S. epidermidis
- found on skin
- rarely suppurative
- may infect prostheses
Pathology
Abscess
- furuncle, carbuncle, pimple
- focal suppuration necrosis
- coagulase coagulates fibrin around lesion to produce wall
- liquefaction of necrosis occurs
Streptococci
Characteristics
- gram positive
- arranged in chains (Strep throat like necklace)
Enzymes
Haemolysins
- haemolyse RBC's
- complete = ß-haemolysis
- incomplete = Alpha-haemolysis
Streptolysis
- Grp A ß-haemolytic Strep produce
- Streptolysin O
- Streptolysin S
Streptokinase
- converts plasma plasminogen to plasmin
- digests fibrin
Hyaluronidase
- splits Hyaluronic acid
- aids in spreading bacteria
Erythrogenic Toxin
- causes rash of scarlet fever
ß-Haemolytic
- Produce Haemolysins
Group A
- S Pyogenes
- majority of pathogens
Cause
- erysipelas
- strep throat
- impetigo
- infective endocarditis
- Rheumatic Fever
- Acute Glomerulonephritis
Group B (B=baby)
- S Agalactinae
- normal flora of female genital tract
- important in neonatal infections
Group G (Remember G=Gut)
- normal enteric flora
- Enterococci / S. faecalis / S. faecium
Alpha-Haemolytic
S Pneumoniae
- Cause pneumonia
S Viridans
- normal respiratory flora
Peptostreptococci
- gut anaerobes
Treatment
All sensitive to Penicillin G
Gram negative Enteric Bacteria
Characteristics
- large heterogenous group
- gram negative rods
- non spore-forming
- facultative aerobes or anaerobes
- natural habitat is GIT
Toxins
1. Endotoxins
Definition
- complex lipopolysaccharides derived from bacterial cell walls
- often released when bacteria lyse
Effects
- act on various cells especially Neutrophils
- cause release of endogenous pyrogens / Acute phase reactants
- stimulate inflammatory response
- activation of complement Cascade with release of vasoactive substances
Activation of coagulation cascade
- hypotension
- early vasoconstriction
- later vasodilatation & increased vascular permeability
- leads to shock / DIC / Metabolic acidosis
2. Enterotoxins
Produce diarrhoea
- E coli (Traveller's diarrhoea)
- Shigella (Dysentery)
- Vibrio Cholerae (Cholera)
Groups
1. Coliforms
- aerobic rods found in GIT
- large & heterogenous group
- all resemble prototype Escherichia coli
- also Klebsiella / Enterobacter / Serratia
- constitute large part of normal aerobic intestinal flora
- become pathogenic when reach tissues outside intestine
- commonest cause of UTI
- various sensitivities
- most sensitive to gentamicin
2. Pseudomonas
- motile aerobic Rods
- widely distributed
- may be found in intestine & skin
- forms blue-green pus & sweetish odour
- common in respiratory tract
- treated with Ceftazidime / Ticarcillin
Also:
- Salmonella
- Shigella
- Vibrio
- Campylobacter
Haemophilus
Characteristics
- Gram negative Bacillus
- Non-encapsulated form is part of normal respiratory flora
- Encapsulated form produces suppurative respiratory infection
Pathology
May enter bloodstream in small children
- meningitis
- septic arthritis
Infants < 3/12 have Maternal Ig
- by 3 years, most children have antibodies
- 3/12 - 3 years risk period
- immunization now available
- HIB has decreased ++++
Treatment
Many susceptible to Amoxicillin
- some produce ß lactamase
Cefotaxime
Neisseria
Characteristics
Gram Diplococcus
- Neisseria Meningitidis / Meningococcus
Pathology
Enter via nasopharynx
- travel via blood stream
Bacteraemia produces
- high fever
- haemorrhagic rash
Clnically
Meningitis
Waterhouse-Friderichsen Syndrome
Sepsis & DIC
Circulatory collapse
Neisseria Gonorrhoeae / Gonococcus
Characteristics
- attach to surface epithelial cells
- attack mucous membranes of Genitourinary Tract / Rectum / Eye / throat
- produces suppuration
Clinically
- urethritis in males
- PID in females
- septic arthritis & OM (secondary to bacteraemia)
- arthritis in knees, ankles & wrists
Treatment
Most serious infections sensitive to Pencillin G
Gradual rise in resistance to Pen G
- use tetracycline