Staphylococcal Skin Infections

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Information about Staphylococcal Skin Infections

Published on February 20, 2014

Author: ArchaDave



Staphylococcal Skin Infections , Organism growth characteristics on Media, Biochemical Characteristics, Virulence Factors, Transmission, Epidemiology

STAPHYLOCOCCAL SKIN INFECTIONS Archa Dave M.Sc Microbiology III Semester 12031G1901

Introduction Staphylococcus aureus: • • • • 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.

Staphylococcus aureus As seen under electron Microscope..

Growth Characteristics on Culture Media

Culture Media 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 (2) (3) (4) (5) (6) : 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 medium 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 acidic conditions. • 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 • • • Staphylococcus aureus Produces yellow colored colonies as it ferments Mannitol Fermentation causes change in color of indicator – Phenol red , from red to yellow. • • • Staphylococcus epiderdimis Non- Mannitol fermenter, hence colorless colonies with pink colored media No change in color of indicator is seen.

Phenyl Ethyl Alcohol Agar Phenylethyl alcohol (PEA) agar with 5% sheep blood inoculated with Staphylococcus aureus, a gram-positive bacterium, shows good growth.

Baird-Parker Agar • 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 MRSA.

Columbia Agar • Columbia agar with 5% defibrinated sheep blood. • Individual colonies on agar are round, convex, colourless and 1-4 mm in diameter with a sharp border.

Blood Agar Complete Hemolysis Partial Hemolysis Different types of Hemolysis

Nutrient Agar • • • • • • • • circular, entire margin, convex, moderate to large rough, shiny, tan to golden yellow opaque.

Biochemical Characteristics

Name of the test Result Indole Test -ve Methyl Red -ve Voges-Proskauer -ve Citrate Utilization +ve Hydrogen Sulphide Test -ve Urease Test +ve Lactose Fermentation +ve Sucrose Fermentation +ve Glucose Fermentation +ve Mannitol Fermentation +ve Lipid Hydrolysis +ve Starch Hydrolysis -ve Gelatin Activity -ve Catalase Test +ve

Virulence Factors

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 poisoning . • 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 (TSS) • 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 TSS Boils and Pimples Septicemia 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.

Diagnosis : • 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 antibiotics. • 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 environment.

Prevention • • 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: Crowding Contact (Frequent skin-to-skin) Compromised skin (cuts or abrasions) Contaminated items and surfaces Lack of Cleanliness.

Epidemiology • • • • • • • (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 outpatients. 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.

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