important bacteria

Staphylococci

morphology

Organisms

· G+ spherical cells

· arranged in irregular clusters

· pairs, chains, grapes

· non-motile and non-spore forming

Culture

· grow on most media in aerobic conditions

· Staph aureus is golden

· Staph epidermidis is white

Growth

· resistant to drying and heat

· resistant to many antimicrobials

1. b-lactamase production

· plasmid mediated

· resistant to some penicillins and cephalosporins

2. methicillin resistance

· independent of b-lactamase production

· mechanism unknown

· function of cell wall structure

3. tolerance

· inhibition without death

· due to lack of activation of autolytic enzymes in cell wall

Toxins and enzymes

Exotoxin

· mixture of materials

· a-haemolysin

· b-haemolysin

· endotoxin C

· are haemolytic and pyogenic

Enterotoxin

· causes vomiting and diarrhoea

· cause of food poisoning

Coagulase

· protein that clots citrated plasma

· produced by Staph aureus (coagulase +ve)

· not produced by Staph epidermidis (coagulase-ve)

· coagulase production considered synonymous with invasive potential

Other

· hyalurodinase

· staphylokinase

· exfoliateive toxin

· ‘toxic shock syndrome’ toxin

pathogenesis

· Staph aureus

· pathogenic and invasive

· Staph epidermidis

· found on skin

· rarely suppurative

· may infect prostheses

pathology

Abscess

· furuncle, carbuncle, pimple

· prototype infection

· causes focal suppuration

· necrosis occurs

· coagulase coagulates fibrin around lesion to produce wall

· liquefaction of necrosis occurs

Food poisoning

· due to enterotoxin ingestion

treatment

· made difficult by

· resistance

· inability of drugs to penetrate abscess

· most staphylococci are penicillin resistant

· many are flucloxacillin resistant

· all are sensitive to vancomycin

· drainage of pus important

Streptococci

morphology

· sperical G+ organisms arranged in chains

· may elaborate capsular polysaccharide that impedes phagocytosis

· pili project from capsule

toxins and enzymes

Haemolysins

· haemolyse RBCs

· complete disruption called b-haemolysis

· incomplete disruption called a-haemolysis

· group A b-haemolytic strep produce

· streptolysin O

· streptolysin S

Streptokinase

· converts plasma plasminogen to plasmin

· produces proteolytic enzyme that digests fibrin

Hyaluronidase

· splits hyaluronic acid

· aids in spreading bacteria

Erythrogenic toxin

· causes rash of scarlet fever

classification

B-haemolytic

· produce haemolysins

Group A

· Strep pyogenes

· majority of pathogens

Group B

· Strep agalactinae

· normal flora of female genital tract

· important in neonatal infections

Group G

· includes enterococci

· Strep faecalis

· Strep faecium

· normal enteric flora

Non-b-haemolytic

Strep pneumoniae

· cause pneumonia

Strep viridans

· normal respiratory flora

Peptostreptococci

· anaerobes

· found in gut

pathology

B-haemolytic strep

· commonly cause

· erysipelas

· streptococcal sore throat

· impetigo

· infective endocarditis

· rheumatic fever

· acute glomerulnephritis

treatment

· all sensitive to penicillin G

Neisseria

· Gram negative

· diplococcus (occurs in pairs)

neisseria menigiditis

· meningococcus

· enter via nasopharnyx

· travel via blood stream

· bacteraemia produces

· high fever

· haemorrhagic rash

· may lead to Waterhouse-Friedrichson syndrome

· sepsis

· IDC

· circulatory collapse

· commonest complication is meningitis

· rapid onset and progression

· sensitive to penicillin G

Neisseria gonorrhoeae

· gonococcus

· attach to surface epitheial cells

· attack mucous membranes of

· genitourinary tract and rectum

· eye and throat

· produces suppuration

· leads to

· urethritis in males

· PID in females

· may progress to tissue invasion

· bacteraemia leads to

· skin lesions (haemorrhagic pustules)

· arthritis (knees, ankles and wrists)

· most serious infections sensitive to penicillin G

· gradual rise in resistance to penicillin G

· these treated with tetracycline

Gram-negative enteric bacteria

· large heterogenous group of organisms

· gram-negative rods

· non spore-forming

· facultative aerobes or anaerobes

· natural habitat is intestinal tract of humans and animals

endotoxins

· complex lipopolysaccharides derived from bacterial cell walls

· often released when bactera lyse

Effects

· similar regardless of bacterial origin

Fever

· act on various cells

· esp. neutrophils

· cause release of endogenous pyrogens

Hypotension

· early vasoconstriction (chill)

· later vasodilatation and increased vascular permeability

· leads to shock

· lowered cardiac output

· impaired organ perfusion

· metabolic acidosis

Inflammatory response

· activation of complement cascade with release of vasoactive substances

· activation of coagulation cascade with DIC

Death

· due to

· organ dysfunction

· shock

· DIC

exotoxins

· most common are enterotoxins

· produce diarrhoea

· produced by

· E coli (traveller’s diarrhoea)

· shigella (dysentery)

· vibrio cholerae (cholera)

groups

Coliforms

· aerobic rods found in GIT

· large and heterogenous group

· all resemble prototype Escherichia coli

· other groups are

· Klebsiella

· Enterobacter

· Serratia

· constitute large part of normal aerobic intestinal flora

· become pathogenic when reach tissues outside intestine

· commonest cause of UTI

· various sensitivieis

· most sensitive to gentamicin

Pseudomonas

· motile aerobic rods

· widely distributed

· may be found in intestine and skin

· only is pathogen

· in areas devoid of normal defences

· as mixed infection

· forms blue-green pus and sweetish odour

· common in respiratory tract

· treated with ceftazidime or ticarcillin

Proteus

· motile aerobic rods found in water

· cause chronic UTI

· treated with gentamicin

Others

· Enterobacteriaceae

· Salmonellae

· Shigellae

· Vibrio

· Campylobacter

Haemophilus

· G- coccoid bacillus

· nonencapsulated form is part of normal respiratory flora

· encapsulated form produces suppurative respiratory infection

· may enter bloodstream in small children and produce

· meningitis

· septic arthritis

· infants under age 3 months have maternal immunity

· by age 3 yrs, most children have antibodies

· many susceptible to amoxycillin

· some produce b-lactamase

· treated with cefotaxime

· immunization now available