STAPHYLOCOCCAL SKIN INFECTIONS
Gram positive bacteria that are small, round (cocci) and occur as clusters
appearing like a bunch of grapes.
S. aureus was discovered in Aberdeen, Scotland in 1880 by the surgeon Sir
Alexander Ogston in pus from surgical abscesses.
More than 20 species of Staphylococcus are described in Bergey's Manual, only
Staphylococcus aureus and Staphylococcus epidermidis are significant in their
interactions with humans.
S. aureus colonizes mainly the nasal passages, but it may be found regularly in
most other anatomical locales while, S. epidermidis is an inhabitant of the skin.
Staphylococcus aureus ~ Gram Stained.
Staphylococcus aureus are Gram Positive Organisms, hence they are
stained violet. Observe their characteristic “Grape clusters” arrangement.
As seen under electron Microscope..
A growth medium or culture medium is a liquid or gel designed to support the
growth of microorganisms or cells.
These are classified into six types:
(1) Basal media
: Only basic components .
E.g., : Nutrient broth, Nutrient agar and Peptone water
Enriched media : Enriched usually by adding blood, serum or egg
E.g., : Blood agar and Lowenstein-Jensen media
Selective media : Favour the growth of a particular bacterium by inhibiting the
growth of undesired bacteria.
E.g., : MacConkey agar, Lowenstein-Jensen media, tellurite
Indicator media : Indicates growth of Particular Microbes.
E.g., : Blood agar and MacConkey agar
Transport media : When specimen cannot be cultured soon after collection.
E.g., : Cary-Blair Medium , Stuart Medium
Storage media : Used for storing the bacteria for a long periods.
E.g., : Egg saline medium, Chalk cooked meat broth.
Growth Characteristics of Staphylococci
Staphylococcus spp. will grow on any media.
They are salt tolerant, as most organisms are killed by high NaCl conc.
"Salt agar" is 7-10% salt containing broth and agar or salt cooked meat broth.
Indicator media containing Phenol Red turns Staphyloccous colonies Yellow in
• When growing “specific“staphs on MacConkey’s agar, the colonies are pink due to
the neutral red indicator (pink in acid, colourless in alkaline)
Various different Agar media are used to culture Staphylococcus aureus; They are:
• Mannitol Salt Agar
• Phenyl ethyl Alcohol Agar
• Baird-Parker Agar
• Blood Agar
• Nutrient Agar
• Columbia Agar
Mannitol Salt Agar
Produces yellow colored colonies as
it ferments Mannitol
Fermentation causes change in color
of indicator – Phenol red , from red
Non- Mannitol fermenter, hence
colorless colonies with pink colored
No change in color of indicator is
Phenyl Ethyl Alcohol Agar
Phenylethyl alcohol (PEA) agar
with 5% sheep blood inoculated
with Staphylococcus aureus, a
gram-positive bacterium, shows
• Medium contains lithium chloride and
tellurite to inhibit the growth of
accompanying microbial flora;
• While pyruvate and glycine selectively
stimulate the growth of staphylococci.
• Zones and rings are formed as a result of
lipolysis and proteolysis
• Reduction of tellurite to tellurium
produces a black colouration.
• This will not differentiate a MSSA from a
• Columbia agar with 5% defibrinated sheep
• Individual colonies on agar are round,
convex, colourless and 1-4 mm in diameter
with a sharp border.
Different types of Hemolysis
moderate to large
tan to golden yellow
Name of the test
Hydrogen Sulphide Test
Enterotoxins and Toxic Shock Syndrome Toxin
S. aureus secretes two types of toxin with superantigen activity,
• Enterotoxins - there are six antigenic types (named SE-A, B, C, D, E and G), and
• toxic shock syndrome toxin (TSST-1)
• Enterotoxins cause diarrhea and
vomiting when ingested and are
responsible for staphylococcal food
• enterotoxins B and C cause 50% of
non-menstrual cases of TSS.
• TSST-1 is expressed systemically and
is the cause of toxic shock syndrome
• TSST-1 is responsible for 75% of TSS,
including all menstrual cases.
TSS can occur as a sequel to any staphylococcal infection if an
enterotoxin or TSST-1 is released systemically and the host lacks
appropriate neutralizing antibodies.
What does S. aureus cause?
Of the variety of manifestations S. aureus may cause:
Minor skin infections, such as pimples, impetigo etc.
It may cause boils (furuncles), cellulitis folliculitis, carbuncles
It is the cause of scalded skin syndrome and abscesses
It may lead to lung infections or pneumonia
Brain infections or meningitis
Bone infections or osteomyelitis
Heart infections or endocarditis
Generalized life threatening blood infections or Toxic shock syndrome (TSS),
bacteremia and septicaemia
Infections caused by Staphylococcus aureus
Boils and Pimples
Surgical wound infections
Scalded skin syndrome
Transmission of Staphylococcus aureus
• S. aureus may occur commonly in the environment. S. aureus is transmitted through air
droplets or aerosol.
• When an infected person coughs or sneezes, he or she releases numerous small droplets
of saliva that remain suspended in air. These contain the bacteria and can infect others.
• Another common method of transmission is through direct contact with objects that are
contaminated by the bacteria or by bites from infected persons or animals.
• Approximately 30% of healthy humans carry S. aureus in their nose, back of the throat
and on their skin.
• When S. aureus is isolated from an abscess or boil or other skin lesion, it is usually due to
its secondary invasion of a wound rather than the primary cause of disease
• leads to pyogenic (abscessing) infections of the skin, eyes and genital tract.
• On culture, the bacterial colonies form a characteristic glistening ,opaque,
yellow to white appearance on blood agar.
• Biochemical tests
Resistance of Staphylococci to Antimicrobial Drugs
• Hospital strains of S. aureus are usually resistant to a variety of different
• A few strains are resistant to all clinically useful antibiotics except vancomycin
• The term MRSA refers to Methicillin resistant Staphylococcus aureus.
• Methicillin resistance is widespread and most methicillin-resistant strains are
also multiply resistant.
• S. aureus exhibits resistance to antiseptics and disinfectants, such as
quaternary ammonium compounds, which may aid its survival in the hospital
The best preventive method is by maintaining good hygiene and regular and
frequent hand washing.
According to the Centre for Disease Control and Prevention (CDC), staphylococcal
infections, including MRSA, occur most frequently among persons where the 5 C's
are present. These include:
Contact (Frequent skin-to-skin)
Compromised skin (cuts or abrasions)
Contaminated items and surfaces
Lack of Cleanliness.
(Data as per 2010 Statistics)
The prevalence of MRSA in Chennai was reported as 40-50 %
S. aureus constituted 17 per cent of catheter related blood stream infections
(CRBSIs) in that centre.
A high prevalence of MRSA (35% in ward and 43% in ICU) was observed from
blood culture specimens in a study in Delhi.
MRSA isolation rates from ICU and wards were higher than that seen among
There is a change in the blood stream infections with S. aureus emerging as the
predominant pathogen in recent years.
In a study from north India, the prevalence of MRSA was 46 per cent and MRSA
isolates were found to be more resistant to other antibiotics than MSSA.
Significant difference was observed in case of erythromycin, ciprofloxacin,
gentamicin and amikacin.