bacterial skin infection lab diagnosis

2. Staphylococcus
•Gram-positive spherical cocci
arranged in grape-like clusters.
•Facultative anaerobe.
•Non motile, non capsulated.
•CATALASE POSITIVE
Staphylococci

3. Staphylococci are divided into:-
•Coagulase positive staphylococci:- S.aureus (most
pathogenic species).
•Coagulase negative staphylococci: -S.epidermidis
and S .saprophyticus are the most important coagulase-
negative species that can cause infections in man.

4. Coagulase positive Staph
(S.aureus)
Coagulase Negative Staph
S.epidermidis
&S.saprophyticus
Growth on
nutrient agar
Golden yellow colonies S.epidermidis: white colonies
S.saprophyticus :lemon
yellow colonies
Catalase test Positive Positive
Coagulase test Positive Negative
Hemolysis on
blood agar
β-haemolytic Non haemolytic
Mannite
fermentation
Fermenter Non fermenter
Gelatin
liquefaction
Liquefy gelatin Not Liquefy gelatin
Pathogenicity Pathogenic opportunistic pathogens
Difference between Coagulase positive Staph & Coagulase Negative Staph

5. Novobiocin sensitivity test:
Is done to differentiate between the coagulase negative
staphylococci: S.epidermidis (Novobiocin sensitive) and
S.saprophyticus (Novobiocin resistant).
S.epidermidis
S.saprophyticus

6. I -Pyogenic
staphylococcal Diseases
a- Focal suppuration
folliculitis, carbuncles, boils
and abscesses.
II. Toxigenic
staphylococcal diseases
Scalded skin syndrome
Diseases caused by staphylococcal

7. b- Invasive conditions:
bacteraemia,osteomyelitis.
c- Outbreaks of hospital acquired post-
operative wound infections
Diseases caused by staphylococcal

8. •Specimens: according to site of infection may include pus,
sputum, urine, CSF, blood ....etc.
• Direct detection in clinical specimens by:
Gram-stained smear: Gram-positive cocci
are seen in clusters in association with pus cells.
Laboratory diagnosis of S. aureus infection:

9. 3. Cultivation:
•On nutrient agar: endopigment.
•On blood agar: β-haemolysis.

10. •On mannitol salt agar: (selective media for staphylococci),
colonies are yellow due to change in colour of phenol red
indicator as a result of acid production from mannitol

11. Biochemical reactions:-
•Catalase test: positive.
•Coagulase test: positive.

12. •Phage typing: To trace the source of
infection in nosocomial infection and
outbreaks of food poisoning.
•Typing of staphylococci is done by
flooding the surface of an agar with broth
culture of the test strain (staphylococcus)
then after dryness different phages are
applied as drops on different square areas
on the plate, which is incubated at 37°C
for 24 hours, lysis will occur in the area
which determine the phage type of the
strain.

13. STREPTOCOCCI
 Streptococci are gram
positive cocci.
 Arranged in chains or
pairs.
 All streptococci are
catalase negative.
 Grow on enriched media.
 Facultative anaerobes.
13

14. Streptococcal classification
14

15. •A local infection of the
skin characterized by
formation of blisters
which break leaving a
denuded surface covered
with pus or crusts.
•May be followed by acute
glomerulonephritis
Impetigo
15

16. 16

17. Scarlet fever

18. •Specimens: Swabs from the lesions
Diagnosis of streptococcal infections
18

19. Diagnosis of Streptococcal Diseases:
•Direct smears stained by gram show gram
positive cocci in chains.
Gram stained film of
Streptococci in culture Gram stained film of
Streptococci in pus
19

20. Cultures :
Done on blood agar incubated at 37°C. Colonies
producing complete haemolysis,.
 Catalase negative .
B. hemolysis on blood
agar 20

21. Bacitracin sensitivity test is done by placing a
bacitracin disc on the inoculum of the organism on
blood agar. A zone of inhibition around the disc is
observed in case of group A streptococci
21

22. •The most important pathogenic member in this genus is
It causes anthrax in man and animals.
Malignant pustule

23. Morphology:
•The typical cells, measuring
•3-4 um have square ends
•and are arranged in long chains.
Gram positive B.anthracis

24. •The bacilli are capsulated in vivo, when
stained with polychrome methylene blue
the capsule appears as pink rim around
the blue bacillus (Macfadyean reaction).
•.
B.anthracis
Macfadyean reaction
(polychrome Methylene blue stain)

25. •The bacilli form spores in vitro, the spores are
central, ovoid and are not stained with Gram, and they
appear pink when stained with the acid fast spore
stain.
•The bacilli are non motile

26. Culture Characters:
•Aerobes grow on nutrient agar.
•Colonies are non haemolytic
•Gelatin is liquified and growth
in gelatin resembles an inverted
fire tree.
Colonies are non haemolytic
Gelatin liquefaction

27. :
Anthrax is a disease of animals as cattle and sheep,
man is infected accidently by contact with diseased
animals causing cutaneous, pulmonary and intestinal
anthrax.

28. •Film stained with Gram
•for characteristic morphology.
•Cultures on nutrient agar, blood agar
and gelatin liquifaction