5. Laboratory diagnosis of Staphylococcal Infections:
Collection of specimens
Pus:
Closed lesions (abscesses):
• surgical collection:
– rigurous cleaning and disinfection of skin (iodine)
– Incision and aspiration of pus
Open lesions:
• Cleaning and disinfection of skin around lesion (iodine)
• Collection of pus with sterile swab / loop
8. Laboratory diagnosis of Staphylococcal Infections:
Collection of specimens
Fluid from cavities
e.g. spinal (CSF)/ pleural / pericardic / articular
• Sample collected by punctioning the cavity
• E.g. Lumbar punction (spinal tap)
– patient lies on the side, knees pulled up toward chest,
chin tucked downward
– back cleaned and disinfected + local anesthetic
– spinal needle inserted into lower back area
– needle properly positioned, CSF pressure measured
and sample collected in sterile tube
9. Laboratory diagnosis of Staphylococcal Infections:
Collection of specimens
Pharyngeal, naso-pharingeal exudate
Patient:
– in the morning, before feeding, before brushing teeth;
alternatively: at least 4 hours since last meal & teeth
brushing
– No mouth rinse, no chewing gum!
– No antibiotics during the last 7-10 days
Medical staff:
– Wear gloves, face protection (mask, eye
protection/face shield), protective lab coat
10. Collection of pharyngeal exudate
• Dacron or Rayon swab
• Tongue blade & good light
• Insert swab behind uvula
without touching it
• Swab tonsils, posterior
pharynx + lesions (if any)
• Avoid touching tongue,
cheeks, teeth
• Place swab in sterile tube
• Transport to lab (RT/2-8°C)
17. Laboratory diagnosis of Staphylococcal Infections:
Innoculation of culture media
• closed collections / moderately contamnated collection
sites (e.g. nasopharingeal swab) → blood agar
• Highly contaminated biological products (e.g. stool)
↓
Chapman agar - selective medium
(high salt content + mannitol + pH indicator)
WHY?:
– A. Inhibit other germs, favour growth of Staphylococcus
– B. Staphylococcal growth →Fermentation of mannitol →colour of
medium changes from pink to yellow (further identification step)
18. Mannitol Salt Agar (Chapman)
- high salt concentration supports growth
of Staphylococcus / inhibits Streptococcus
- mannitol acidification - turn the medium colour to yellow
29. Laboratory diagnosis of Streptococcal Infections:
Innoculation of culture media
Innoculation on blood agar:
• Colonial morphology:
– Str.pyogenes: small, pinpont, 0.5 μM diameter, transparent
– Str.pneumoniae, Str.viridans: small, smooth, flat/depressed
center (autolysis)
• Type of hemolysis:
- β-hemolysis - complete digestion of red blood cell contents
surrounding colony e.g. Streptococcus pyogenes
- α-hemolysis - partial lysis – incomplete hemoglobin digestion →
green or brown (conversion of hemoglobin to methemoglobin)
e.g. Streptococcus viridans, Streptococcus pneumoniae