Lab Diagnosis In

Staphylococcal Infections

Deepa Babin

Deepa Babin at TMC Kollam 2

Diseases Caused by Staphylococcus aureus

Infections

Localised pyogenic infections

Folliculitis, furuncle, carbuncle

Abscesses (e.g. Breast abscess)

Umbilical sepsis

Wound infections

Respiratory infections

Upper respiratory tract infections

Sinusitis, otitis media

Lower respiratory tract infections

Lung abscess, empyema, bronchopneumonia

Musculoskeletal

Osteomyelitis

Systemic

Septicaemia, Endocarditis

UTI (Caused by Staphylococcus saprophyticus)

Intoxications

Food poisoning

Staphylococcal scalded skin syndrome (SSSS)

Toxic shock syndrome

Deepa Babin at TMC Kollam 3

Specimens Collected

Depends on type of lesion / disease

Pus (aspirates or swabs)

Sputum (in respiratory infections)

Blood (in endocarditis,

septicaemia)

Pleural fluid (in empyema)

Nasal swab (detection of MRSA

carriage)

Deepa Babin at TMC Kollam 4

Direct Microscopy

Gram stain of pus

Pus cells with Gram

positive cocci (GPC) in

groups

Pus cells

GPC in groups

Deepa Babin at TMC Kollam 5

Culturing of Staphylococcus

Media used:

Blood agar, chocolate agar, MacConkey agar

Mannitol salt agar (Selective / Differential medium)

Liquid media: Glucose broth, brain-heart infusion broth

Colony characteristics

On blood agar: Golden yellow, opaque, β-lytic

colonies

On mannitol salt agar: yellow colonies (mannitol

fermenting)

Deepa Babin at TMC Kollam 6

Identification of Isolate

Gram smear examination

Positive catalase test (to distinguish from streptococci)

Positive coagulase test (to distinguish Staphylococcus aureus from other staphylococci) Slide coagulase test

For detection of bound coagulase

Make a saline suspension of the organism on a clear glass slide; add a drop of human plasma

Immediate clumping indicates a positive reaction

Tube coagulase test 0.1ml of overnight broth culture of

organism is mixed with 0.5ml of citrated / oxalated / heparinised human plasma in a test tube

Tube is incubated at 37ºC for 3–6 hours in a water bath

A plasma clot indicates the presence of free coagulase

Positive DNase test (to distinguish Staphylococcus aureus from other staphylococci)

Tube Coagulase Test

+

–

Deepa Babin at TMC Kollam 7

Antibiotic Sensitivity Testing

I line

Penicillin (Ampicillin in urinary isolates)

Gentamicin

Erythromycin (not used in urinary isolates)

Cephalothin (I generation cephalosporin)

Amoxicillin-Clavulanate

Oxacillin (or Cloxacillin,cefoxitin)

Tetracycline

Nitrofurantoin (only in urinary isolates)

II line

Vancomycin

Rifampicin

Clindamycin

Linezolid

Netilmicin

Amikacin

Deepa Babin at TMC Kollam 8

Treatment

If Penicillin is sensitive: Penicillin is the drug of choice

If not sensitive Oxacillin / Cloxacillin + an Aminiglycoside

For MRSA Vancomycin or Linezolid

Chlorhexidine or Mupirocin for local application

Treatment of abscesses Drainage is essential + antibiotics

In severe, life-threatening infections (septicaemia, endocarditis, pneumonia,osteomylitis) Oxacillin / Cloxacillin + an aminiglycoside

Deepa Babin at TMC Kollam 9

Coagulase Negative

Staphylococci Staphylococcus epidermidis

Stitch abscess

Infection associated with artificial implants and prosthetics, such as: Central venous line

Intra ocular lens

Ventriculo-peritoneal shunt

Artificial heart valves (endocarditis)

Artificial joints

Staphylococcus saprophyticus Causes UTI in young adult females

Resistant to Novobiocin (as opposed to Staphylococcus epidermidis, which is sensitive)