STAPHYLOCOCCUS

STAPHYLOCOCCUS

(Staphyle-bunch of grapes; kokkus- berry)

• Common cause of suppurative lesions

• Very important because it develops resistance to antibiotics in hospital

TWO TYPES

1.S. aureus

• pus forming & virulent

• Requires heating at 80◦C for 1 hour to kill

2. S. epidermidis

• Non pathogenic & non- virulent

• Present in skin commonly

PATHOGENECITY

• Staphylococcus produces 2 types of diseases: Infections & Intoxications

1. Infections – Gain access through skin, mucosa or tissue sites, colonise, evade host defence, multiply & tissue damage.

2. Intoxication- caused by the bacterial toxins in infected host or in vitro.

INFECTIONS

Following are the main virulence factors of staphylococcus:

I. Cell associated polymers:

1. Peptidoglycan in cell wall gives the structure & rigidity to the bacterial cell; releases inflammatory cytokines

2. Teichoic acid facilitates adhesion to host cell wall

3. Capsular polysaccharide surrounding cellwall inhibits opsonisation

II. Cell surface proteins

1. Protein A – mainly chemotactic and phagocytic in nature; induces platelet damage & hypersensitivity

2. Clumping factor-surface protein ‘bound coagulase’ is responsible for the ‘slide coagulase test’

III. Extracellular enzymes:

1. Coagulase, an enzyme which clots ‘only’ human or rabbit plasma

2. Lipases (Hyaluronidase & staphylokinase) infects skin and connective tissues

3. Nuclease, a heat stable nuclease is a characteristic feature of staphylococcus aureus

4. Protein receptors

INTOXICATION

• 1. Cytolytic toxins – 4 hemolysins & 1 leucocidin

• α- hemolysin – most important.

-Inactivated at 70◦C, but reactivated at 100◦C. Because, at 60-70◦C, the toxin combines

with a heat labile inhibitor which is denaturated at 100 ◦C, leaving the toxin free.

• β -hemolysin – haemolytic for sheep cells, not for humans or rabbits

• γ -hemolysin

• δ -hemolysin

• Leucocidin

2. Enterotoxin

• Responsible for staphylococcal food poisoning; nausea, vomiting & diarrhoea 2-6 hours after consuming the contaminated food.

• Toxin is heat stable, resisting 100 ◦C for 10-40 minutes

• Meat and fish or milk or milk products ‘cooked’ & left at room temperature for long time

are the common cause

• Source of infection is generally food handler (carrier)

• Recovery in just a day or two

• 3. Toxic Shock syndrome Toxin (TSST):

• It is a potentially fatal multisystem disease presenting with fever, myalgia, hypotension, vomiting, diarrhoea, mucosal hyperaemia and erythematous rash.

4. Exfoliative (epidemolytic) toxin:

• Also known as ‘exfoliatin’ responsible for ‘staphylococcal Scalded Skin Syndrome’ (SSSS)

STAPHYLOCOCCAL DISEASES

- Most common of bacterial infections ranging from trivial to fatal

- Its localised, unlike streptococcal infections which spreads

1. Skin & soft tissue – folliculitis, furuncle (boils), carbuncle, abscess, wound infection

2. Musculoskeletal – osteomyelitis, arthritis, bursitis

3. Respiratory- tonsillitis, pharyngitis, sinusitis, otitis, emphyema

4. CNS- abscess, meningitis

5. Endovascular- bacteraemia, septicaemia, pyemia, endocarditis

6. Urinary- Uncommon

MODE OF TRANSMISSION

• Common source- Human beings

• Patients with superficial & respiratory infection spread a lot

• 10-30% of healthy adults carry it in nose

• Common in umbilical stump of new born

• Shedders in hopsitals- handkerchief, towels & bed linens

• Few bacterias called ‘hospital strains’ develop resistance easily

• Main reason for hospital acquired wound infection and other cross infections

• Control measures

- Isolate the infected patients

- Screen hospital staffs during outbreak

- Strict aseptic hospital theatres

Most importantly,

- Wash hands

LAB DIAGNOSIS

• Wound infection

• Nasal Swab

• Sputum samples

• Under microscope, shows cluster of bacteria. Not visible in sputum sample due to contamination

• Diagnosis easier in culture, as it grows in overnight incubation.

• If scanty numbers expected, specimen are inoculated in media like ludlam’s or salt-milk agar or Robertson’s cooked meat medium, with 10% NaCl and coagulase test done with the staph extract.

• Coagulase test- 2 types

• 1. Tube coagulase: 0.1 ml of agar culture of specimen + 0.5ml of human plasma in a test tube with EDTA or heparin.

- Tube is incubated in water bath for 37 ◦C for 3-6 hours

• 2. Slide coagulase test: The isolate is emulsified in a drop of saline on a slide & a drop of human plasma is added to emulsion & mixed.

Clumping indicates it is positive