Microbes - Specific

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