Staphylococcus aureus

Staphylococcus aureus

GENERAL CHARACTERISTICS

·       Gram’s classification – Gram positive bacteria

·       Shape – Spherical (Cocci)

·       Arrangement – Bunched or Grape like clusters

·       Motility – Non-Motile

·       Capsule - Absent

·       Endospores - Absent

·       Respiration – Aerobic/Facultative anaerobic

·       Optimum Temperature - 37 °C

·       Optimum pH – 7.0 to 7.5

·       Normal human flora on skin and mucosal surfaces.

·       The organisms are ubiquitous, so there are no Geographic or seasonal limitations.

·       Discovered by Anton J. Rosenbach (German Surgeon) in 1884.

PATHOGENICITY OF Staphylococcus aureus

DISEASE TRANSMISSION

            Staphylococcus aureus infections are mainly transmitted by

a)     Direct contact with an infected person

b)     By using a contaminated object

c)     By inhaling infected droplets dispersed by sneezing or coughing.

INCUBATION PERIOD

1 to 6 Hours

VIRULENCE FACTORS OF Staphylococcus aureus

(i)    Structural components

a)     Capsule

b)     Slime layer

c)     Peptidoglycan

d)     Teichoic acid

e)     Protein A

(ii)  Toxins

a)     Cytotoxins

b)     Exfoliative toxins (ETA & ETB)

c)     Enterotoxins (A to R)

d)     Toxic Shock Syndrome Toxin 1

(iii) Enzymes

a)     Coagulase

b)     Hyaluronic acid

c)     Fibrinolysin

d)     Lipases

e)     Nucleases

f)      Beta Lactamase

g)     Staphylokinase

PATHOGENESIS OF Staphylococcus aureus

·       Staphylococcus aureus enters into the surface of the Skin, grows into Hair follicles (The sheath of cells and connective tissue which surrounds the root of a hair) and invades Sebaceous glands (Exocrine glands in the skin that secrete an oily or waxy matter, called Sebum, to lubricate and waterproof the skin and hair). After that, it triggers Fever and Inflammation, which are natural responses against infection, and causes the follicle to enlarge and fill with pus composed of leukocytes, dead cells, and bacteria.

·   The infection may spread into the Hypodermis to form a Furuncle or into neighboring hair follicles to form a Carbuncle.

·    Staphylococcus aureus may also spread into the blood - a condition called Bacteremia and is carried to the lining of the heart, lungs, and bones, causing Endocarditis, Pneumonia and Osteomyelitis, respectively.

CLINICAL DISEASES CAUSED BY Staphylococcus aureus

I) TOXIN – MEDIATED DISEASES

(i)    Staphylococcal Scalded Skin Syndrome (SSSS)

·      Also known as Ritter von Ritterschein disease (in newborns and under the age of 5), Ritter disease, and Staphylococcal epidermal necrolysis.

·  Superficial blistering skin disorders caused by the Exfoliative toxins of some strains of Staphylococcus aureus.

·      Exfoliative toxin that causes the outer layers of Skin to Blister (a small pocket of body fluid containing lymph, serum, plasma, blood or pus) within the upper layers of the skin) and Peel.

(ii)  Staphylococcal Food poisoning

·    Staphylococcal Food Poisoning results after consumption of food contaminated with heat-stable Enterotoxin.

·     Symptoms typically come on quick. It include Nausea, Vomiting, Abdominal cramping and Diarrhea.

·   In more severe cases, Dehydration, Headache, Muscle cramping, and changes in Blood pressure and Pulse rate may occur.

·      The condition is typically over in 2 days. But it is not unusual for complete recovery to take 3 days and sometimes longer in severe cases.

(iii)  Toxic Shock Syndrome (TSS)

·     Toxic Shock Syndrome (TSS) is a rare but serious medical condition caused by a bacterial infection. The bacterium Staphylococcus aureus gets into the blood stream and produces Toxins.

·    Symptoms of Toxic Shock Syndrome (TSS) are Sudden high fever; Low blood pressure (Hypotension); Vomiting; Diarrhea; Rash resembling a Sunburn; Confusion; Muscle aches; Redness of eyes, mouth & throat; Seizures (Sudden, uncontrolled electrical disturbance in brain);Headaches; Shock; Renal failure and sometimes Death.

II) SUPPURATIVE INFECTIONS  

(i)    Impetigo

·   The most common cause of impetigo is bacteria called Staphylococcus aureus. Another bacteria source is group A Streptococcus.

·     Impetigo is a common and highly contagious skin infection that mainly affects infants and children.

·     Impetigo usually appears as red sores on the face, especially around a child's nose and mouth, and on hands and feet.

(ii)  Folliculitis - Impetigo involving hair follicles

(iii)  Furuncles or Boils -    Large, painful, pus-filled cutaneous nodules.

(iv) Carbuncles – Coalescence of Furuncles with extension into the subcutaneous tissues and evidence of systemic disease (fever, chills, bacteremia)

(v)  Bacteremia - Spread of bacteria into the blood from a focus of infection.

(vi)   Endocarditis – Endocarditis characterized by damage to the endothelial lining of the heart.

(vii) Pneumonia and Empyema - Consolidation and abscess formation in the lungs; seen in the very young and elderly and in patients with underlying or recent pulmonary disease; a severe form of necrotizing pneumonia with septic shock and high mortality is recognized.

(viii) Osteomyelitis - Destruction of bones, particularly the metaphyseal area of long bones.

(ix) Septic arthritis – Painful erythematous joint with collection of purulent material in the joint space.

LABORATORY DIAGNOSIS OF Staphylococcus aureus

MICROSCOPIC EXAMINATION

·     Gram stating – Violet coloured Gram positive cocci arranged in form of clusters.

·     Motility test – Non-Motile.

COLONY MORPHOLOGY ON CULTURE MEDIUM

·     Nutrient agar - Smooth, golden yellow colonies.

·     Blood agar – Beta Hemolysis.

·     MacConkey agar – Pink coloured Lactose fermenting colonies.

·     Selective medium

ü  Mannitol Salt Agar (MSA) – Yellow to Golden yellow colonies.

BIOCHEMICAL TESTS

a)     Catalase test - Positive

b)     Oxidase test - Negative

c)     Urease test - Negative

d)     Indole test - Negative

e)     Methyl Red (MR) test - Negative

f)      Voges Proskauer (VP) test - Positive

g)     Citrate utilization test – Positive

h)    Starch Hydrolysis – Negative

i)      Casein Hydrolysis - Negative

j)      Coagulase Test – Positive

k)     Tellurite Reduction Test – Positive

l)      DNase Test - Positive

 ANTIBIOTIC THERAPY AND TREATMENT

• The antibiotics of choice are Oxacillin (or other Penicillinase - resistant Penicillin) or Vancomycin for Oxacillin-resistant strains.
• Removal of the foreign body is often required for successful treatment.

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