Staphylococcus aureus

Staphylococcus   aureus

                                  

-Etymology :

  Staph    : Greek word Meaning “ Grape like cluster”

  Coccus : berry (spherical shape)

  aureus :  Latin word meaning “ Golden” (gives golden  yellow color colonies )    

   

- Discovery             

S. aureus was discovered by the surgeon Sir Alexander Ogston (1880), Scotland in pus from surgical abscesses 

-Habitat: Ubiquitous in nature and normal inhabitant of the skin and nose( mucous membranes ) of health persons. S. aureus is infectious to both animals and humans and may only survive on dry skin. Spread through contaminated surfaces, air and people.

                                          Morphology 

-Gram Positive Organism 

- Cocci -  O -  in shape 

- Size : 1 to 2  μm in diameter

- Non- motile

- Non- Spore forming

- Appear in single, pair and short chains

- Some are have capsule. 

                                         Cultural characteristics

-          -Grow on ordinary medium eg. nutrient agar

-        -  Growth temperature is 10 ^° C  - 42^° C

-         - Optimum temperature is 37^°C

-         - Aerobes and facultative anaerobes

-         - pH range 7.4 to 7.6

-          -Nutrient agar :   It produces Circular, smooth, Convex and opaque colonies.

-         - Blood agar :  It produces marked hemolytic (β- hemolysis) colony. (Sheep blood)

-          -Mannitol Salt agar (MSA) : It produce yellow colonies with yellow zones, whereas   other Staphylococci  produce pink or red colonies without color change of the medium

-          -Liquid medium :  Uniform turbidity

-          -Selective medium: Salt milk agar ,Salt broth

Nutrient Agar

Blood Agar

 

Mannitol Salt Agar (MSA)

Salt Milk Agar

                                                                         Biochemical Test

Catalase Test: Positive

Coagulase Test: Positive

Coagulase is an enzyme-like protein and causes plasma to clot by converting fibrinogen to fibrin in palsma. 

Carbohydrate fermentation : S. aureus ferment mostly  mannitol sugar and produce acid without gas.

                                           Pathogenicity

Infections & Diseases

Skin & Soft Tissue:  Boils, Abscess, Wound infection, Impetigo, Cellulitis, Folliculitis

Musculoskeletal:  Osteomyelitis, Arthritis, Pyomyositis

Respiratory: Tonsillitis, Pharyngitis, Sinusitis, and Bronchopneumonia

Central nervous system:  meningitis, abscess

Endovascular: Bactremia, Septcemia, Endocarditis

Urinary Tract Infection : Uncommon

TSS: Toxic Shock Syndrome (Multisystem disease) symptoms are Fever, Vomiting, Diarrhea, Rashes

SSSS: Staphylococcal Scaled Skin Syndrome is Outer layer of epidermis get separated. 

Virulence Factors

Cell surface proteins:

Protein A: Anti- phagocycotic, Anti- complimentary and damages the platelets.

                  Clumping factor: in slide test coagulate the plasma

Extra Cellular Enzymes: Lipases (Hydrolysis the lipids),

                                          Hyaluronidases (Breakdown the Hyaluronic acid present in the  

                                        tissues),

                                        Nucleases (Breakdown nucleic acid)

Toxins:      Cytolytic cytotoxins; 

                            hmolysins ,

                            Leucocidine

                   Enterotoxin: Causes Staphylococcus food poisoning

                                      Types are A, B, C_1-3, D, E & F

                                       Present in Mil products, meat etc., 

Laboratory Diagnosis

Collection of Samples:  Skin Scrapping, Pus, Food samples, Throat and nasal swabs.

Microscopic observation: Gram staining

Culture: Nutrient agar, Blood agar, Mannitol Salt agar (MSA)

Biochemical Test: Catalase Test,

                              Coagulase Tests (Slide and Tube Test)

Antibiotic Sensitivity Test  ( Kirby bauer Method ) :

            Drugs  : Penicillin,

                          Methicillin (Some strains are resistant against Methicillin are called MRSA)

                          Coaxacillin,  Bacitracin, Vancomycin, Rifampicin, Cephalosporins ,                    

                           Erythromycin, Tetracyclines  etc.

Other diagnosis methods: 

             Phage Typing: Phage typing is a method used for detecting single strains of bacteria. It is used to trace the source of outbreaks of infections. The viruses that infect bacteria are called bacteriophages.

            Molecular Typing : Using RFLP, RAPD, MLST , REP-PCR, Real time PCR 

                 MALDI-TOF:  emerging technique to identify the unkown bacteria