Transcript the gram positive cocci

Diagnostic microbiology
lecture: 8
THE GRAM POSITIVE COCCI
Abed ElKader Elottol
MSc. Microbiology
2010
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STAPHYLOCOCCI
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Species
33 species are known.
Three are medically important:
1. Staphylococcus aureus
2. Staphylococcus epidermidis
3. Staphylococcus saprophyticus
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Most important pathogen
May cause endocarditis
May cause cystitis.
General Characteristics
1. Cocci arranged in grape-like clusters
2. Strongly gram-positive
3. Ferments many carbohydrates with the production of lactic
acid but no gas
4. Non-motile
5. Non-spore forming
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Staphylococcus aureus
• Aureus: golden color (golden color colonies on blood agar)
• Diseases caused by the organism:
1. Toxic shock syndrome
2. Furuncles (abscess)
2. Septicemia
4. Impetigo
5. Meningitis
6. Pneumonia
7. Food poisoning
8. Pyoderma
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Cultural and Morphological Characteristics
1. Media for Primary Isolation:
S. aureus grow well in most routine media e.g,Blood Agar,
Nutrient Agar.
2. Media for Selective Isolation:
Mannitol Salt Agar (MSA), is an excellent medium which
contains 7.5% sodium chloride which is considered as a high
percentage and inhibitory to most medically important
bacteria.
In addition it contains mannitol as the only carbon source and a
pH indicator to detect mannitol fermentation by S. aureus.
3. Incubation:
After streaking the specimen on one of the common media,
incubate the plates at 35-37 oC for 24 hours.
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Colony Morphology
• On blood agar plates:
colonies are 2-4 mm in diameter, rounded and slightly elevated.
Most pathogenic strains produces a zone of β-hemolysis.
Another distinguishing character is the production of a golden
yellow pigment.
• On Mannitol Salt Agar:
The colonies are surrounded by a yellow zone indicative of acid
production resulting from the fermentation of Mannitol.
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Gram Stained Smears:
• Gram-positive cocci arranged in clusters.
• Single cells, diplococci, and short chains may also appear.
• It is usually simple to identify the morphology in stained film
from sputum or pus but one can be certain by performing
simple biochemical tests for the isolate e.g.,
• catalase test to differentiate it from Streptococci and
Coagulase or DNase to differentiate it from non-pathogenic
staphylococci..
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Biochemical Characteristics
1. Catalase positive
2. Mannitol fermenter
3. Grow well in 7.5% NaCl
4. Coagulase positive
5. DNase positive
6. Glucose fermenter.
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Identification:
1. Based on gram-staining
2. To differentiate it from other gram-positive cocci
(Streptococci) perform catalase test
3. To differentiate it from other non-pathogenic
staphylococci a group of biochemical tests are
performed.
a. Coagulase test
b. Mannitol fermentation
c. Growth in 7.5% NaCl
d. Glucose OF
e. DNase
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PHAGE TYPING
• A Staphylococcus aureus-specific
phage is added to a plate that is
inoculated with S.aureus.
• The plates are incubated at 37
oC for 24 hours.
• Positive identification:
Formation of plaques
• Negative results:
S. aureus grow over the whole
area.
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Sensitivity testing and treatment
• S. aureus is a frequent hospital pathogen and it has the
ability to develop resistance to the commonly known
antibiotics.
• For this reason sensitivity testing must be performed on
all isolates.
• Penicillin G and its derivatives (ampicillin, amoxycillin,
cloxacillin, methicillin), ofloxacillin and cephalosporins
are usually effective against S. aureus.
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ANTIBIOTICS RESISTANCE
Historical aspect
• 1940s : all S. aureus were sensitive to penicillin
• Shortly after use : penicillin resistant strains appeared
which produced betalactamase rapidly spread
• In late 1950s : beta-lactamase - resistant penicillin
(methicillin) (not degraded by)
• In 1961 methicillin-resistant S. aureus (MRSA) was
discovered (presently a major problem)
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MRSA
• Low carriage rate in community
• High in tertiary care hospitals
Mode of Transmission
• Fomites
• Direct from hospital staff or attendants : contaminated hands
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MRSA causes a variety of
disseminated, lethal infections in
humans.
• Has the ability to easily transfer
resistant genes to other species
directly and indirectly .
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Overuse of antibiotics imposes
selective pressures which
mediates the acquisition of
resistance .
• Most major organs fail with
disseminated MRSA .
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PREVENTION OF STAPH INFECTIONS
• Control of Carrier and reinfection
• Wash clothes in hot water (>70oC)
• Use antiseptic soap (Dettol soap)
• Antimicrobial nasal cream (Gentamicin, Mupirocin) Oral
antibiotics that are concentrated in nasal secretions
(ciprofloxacin and rifampicin)
• Chemoprophylaxis :Antibiotics before and at time of surgical
operation
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Negative
Oxidase test
COAGULASE-NEGATIVE STAPHYLOCOCCI
(CNS)
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• Normal flora in
° Skin
° Anterior nose
° External ear canal
• Cell wall contains teichoic acid (glycerol type)
•White, non-haemolytic colonies on blood agar
• Sensitive to novobiocin; (S. saprophyticus is resistant)
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DISEASES BY S. EPIDERMIDIS
• Most infections are hospital acquired
• Opportunistic pathogen in immuno-suppressed
• Strongly associated with presence of foreign bodies
° Prosthetic heart valves (endocarditis)
° IV catheters (bacteremia)
° Urinary catheter (UTI in elderly)
° CSF shunts (meningitis)
° Peritoneal dialysis catheter (peritonitis)
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Staphylococus saprophyticus
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• Saprophytic in life.
• Resistant to novobiocin.
• Most infections are community-acquired.
° Primary UTI in 10-20% of young adult women.
hormonal factors may be involved.
• Resistant to antibiotics – penicillins & cephalosporins
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The End
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