Medical Microbiology

Medical microbiology is a branch of science that deals with the study of of pathogenic micro-organisms and is concern with disease, diagnosis ,prevention, control and treatment.

Diseases caused by micro-organisms

  • Staphylococcus aureus
    Staphylococcus aureus

    Staphylococcus aureus


1. Morophology

  • Staphylococcus aureus are gram-positive spherical cocci approximately 1 micrometer in diameter.These cells may be arranged in singly,in pairs,shorts chains or in clusters.
  • They are non-spore forming, non-motile.They may be capsulated or non-capsulated.
  • Under influence of some chemicals the cocci form may change into L-forms.

2. Cultural characters

Staphylococcus aureus  colonies on blood agar showing zone of heamolysis.

Staphylococcus aureus colonies on blood agar showing zone of heamolysis.

  • Staphylococcus aureus may readily grow in ordinary media.It can grow on Nutrient agar, MacConkey’s agar.
  • It grow on temperature range of 10° C  to 42° C and optimum temperature for its growth is 37° C
  • Optimum pH is 7.3-7.6.
  • They may be aerobic or facultative anaerobic in nature.
  • The colonies are 2-4 mm in diameter,circular in shapes ,convex and shiny .
  • These organisms are golden yellow pigment producing organism.These pigment production occurs in aerobic cultures at optimum pH at 22°C.
  • Most of the strains of Staphylococcus  aureus show heamolytic activity on blood agar.
  • The Staphylococcus aureus show pink coloured colony due to lactose fermentation on MacConkey’s agar.

3. Biochemical characters

  • Staphylococcus aureus  is able to ferment number of sugars by producing acid but no gas.
  • Fermentation of mannitol is useful character for diagnosis of Staphylococcus aureus        it produces acid and no gas in mannitol sugar fermentation.
  • This organism shows catalase, urea hydrolysis, gelatin liquification,MR and VP, phosphatase and co-agulase test positive.
  • It can reduce nitrates to nitrites.
  • It shows indole test negative.
  • Most of the Staphylococcus strains are lipolytic in nature.

4. Resistance

  • Staphylococcus is a non-spore forming resistant bacteria with a viability of 3-6 months.
  • It can be isolated from dried pus.
  • It is a heat resistant bacterial and can grow at temperature of about 44°C to 45° C .
  • It can also tolerate high salt concentration up to 10%  to 15% of NaCl.
  • Staphylococcus can get inhibited with 1% phenol for 15 minutes and 1 % mercuric perichloride for 10 minutes.
  • Staphylococcus is penicillin resistant bacteria.
  • It can be inhibited at temperature of about 80° C for one hour.

5. Pathogenecity and virulence

  • Diseases produced by Staphylococcus are of two types that are infection and intoxication.
  • In infection the Staphylococci get attach to damage skin or tissue and multiply and after multiplication it overcome the host defense mechanism and cause tissue damage.
  • In intoxication the disease occurs by toxin production by organism or by host.

6. Staphylococcus disease

  • It is the most common bacterial infection.
  • Common Staphylococcus infections. 
  1. Respiratory infection.
  2. Skin and soft tissue infection.
  3. Central nervous system infection.
  4. Musculosketal and urinary tract infection.

7. Laboratory diagnosis

  • The sample is collected from the infected area of skin, pus from lesion, sputum sample form respiratory infection.
  • In case of food poisoning suspected food sample is collected.
  • Gram nature and motility is observed.
  • The organism is plated on blood agar and observed for heamolysis.
  • Staphylococcus are inoculated on selective media like Robertson’s cooked medium.
  • Co-agulase test is carried out as it is co-agulase positive.
  • Antibiotic sensitivity test should be carried out for proper treatment.

8. Treatment

  • Staphylococcus is resistant to some antibiotic so antibiotic sensitivity test should be performed and appropriate antibiotic should be choosen.
  • If the strain are sensitive to benzyl than penicillin is the most effective antibiotic.
  • Vancomycin is effective drug in dangerous infection.

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