Medically important Bacteria: Part II- Streptococcus

Introduction

Apart from staphylococcus,just discussed in my previous blog, streptococcus plays a leading role in most of the bacterial infections. Usually,these species are aerobic and are gram-negative in nature. Many are capsulated and capsule is mostly composed of hyaluronic acid.

Determinants of Pathogenicity-

1) Capsule-
Capsule serve an anti-phagocytic activity and serves to prevent phagocytosis by host’s microphages and macrophages.

2) Peptidoglycan-
Peptidoglycan, composed of NANA (N-acetylneuraminic acid), NAGA (N-acetylglucosamine) & N-acetylcarboxyethylglucosamine, serves a pro-inflammatory activity.

3) Endotoxins-
The bacterial LPS (Lipopolysaccharide) is pyrogenic and serves as an endo-toxin. It produces fever in host’s body by directly acting on hypothalamus.

4) Antigens-
There may be 3 types of antigens specified for streptococcus. They are:

1) M Proteins- These are the most virulent type of all the three types of bacterial proteins. They serve to attach to host’s epithelial cells and cause adherence to target cell. The extra-bodily adherence of streptococcus over skin is partly prevented by IgA type antibody.

2) F Proteins- These proteins also serve to bind to host’s epithelial cells. The name is derived from the word ‘fibronectin’, which illustrates that the protein helps in adherence to fibronectin cells.

3) G Proteins- These serve to block the Fc receptor of IgG type immunoglobulin, which inturn prevents the compliment binding and further destructive cascade for the bacteria.

4) Exotoxins-
The most common exotoxins produced by streptococcal species are:

a) Enterotoxins- These are similar to enterotoxins produced by staphylococcus and serve to cause cell death of Gastrointestinal Tract and cause GIT infections,dysentery etc.

b) Haemolysins- These toxins serve to lyse off erythrocytes. They are further of 2 types:

    1) Haemolysin O- These toxins are found and produced deep into the colony of streptococcal species. These are sensitive to oxygen and are oxygen labile.

    2) Haemolysin S- These are found superficially over the surface colonies of bacteria.

5) Enzymes-

1) Streptokinase- This serves to liquefy blood clots and are produced by bacteria to liquefy the clots formed over them in the blood vessels. This enzyme is used medicinally as a thrombolytic and is used in the dose 7.5-15 lakhs IU for coronary blockage and 2.5-5 lakh IU for deep vein thrombosis.

2) Streptodornase- This enzyme is used medicinally to liquefy pus cells and solidified purulent exudates.

3) Hyaluronidase- This enzyme acts against hyaluronic acid found in the capsule and lyses the colonies in conditions of bacterial overpopulation.

4) Deoxyribonuclease- This enzyme serves to act against DNA and destroys the host cells.

Diseases produced-

1) Superficial Infections-

These include-

a) boils and carbuncles

b) pyoderma

c) impetigo

2) Deep Infections- These include deep infections involving various organs like:

a) Kidneys- Pyelonephritis, interstitial nephritis, tubuloglomerular nephritis, glomerulitis etc.

b) CNS- This include meningitis and other neuronal diseases involving Bell’s Palsy leading to facial paralysis.

c) Heart- This includes endocarditis, pericarditis, stenosis etc.

d) Lungs- This includes pleuritis, pneumonia, secondary attack over broncheictasis etc.

Drugs used-
Antibacterial drugs and antibiotics are most commonly used against streptococcus. Few are enumerated as follows:

1) Cloxacillin
2) Ampicillin
3) Streptomycin
4) Erythromycin
5) Sulphathiazole
6) Sulphamethoxazole

Links-

1)www.textbookofbacteriology.net/streptococcus.html

2)www.rockefeller.edu/vaf/strep.htm

3)gsbs.utmb.edu/microbook/ch013.htm

4)books.google.co.in/books?isbn=0306456036...

Conclusion-

This was a short piece of information about streptococcus. This was, in all a really brief idea and huge volumes of facts are available today about streptococcus due to numerous advances in research in infectious diseases. I would like to welcome all sorts of comments and criticism, the readers might have.