ANTIBIOTICS

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ALT CULTURE CLUB TOPIC

ANTIBIOTICS

Antibiotics are substances produced by microorganisms that have the capacity to inhibit the growth of other microorganisms.  Microorganisms differ markedly in resistance and susceptibility to various antibiotics, not only to genera and species but between strains of species.  Thus, information regarding the identity of the pathogen isolated and its in vitro susceptibility are of utmost important.  The term "chemotherapy" refers to the destruction of microoorganisms by drugs.  The terms "antibiotics" and "chemotherapeutic agents" are used interchangeably due to the fact that some of the antibiotics originally isolated from natural source have also been prepared synthetically.  An example of this is chlorampenicol.  Antimicrobial drugs have been used in the treatment of bacterial infections for more than 80 years since the discovery of penicillin (extracted from the fungus Penicillium species) by Sir Alexander Fleming in 1929.  As experience accumulates with the use of these drugs, the extraordinary capacity of the microbes to circumvent the actions of antimicrobial agents become apparent.  For example, pneumococci are among the bacterial species most susceptible to the action of antimicrobial drugs and their susceptibility to these compounds has remained remarkably stable.  Reports of tetracycline-resistant pneumococci indicate, that antimicrobial sensitivity of these organisms must be re-evaluated continually.  Resistant strains have the capacity to produce disease, an in vitro resistance can be correlated with failure of tetracycline in vivo.  The reasons for the late appearance of tetracycline-resistant strains of pneumococci and the mechanism involved are unknown.  The exact mechanism of the development of increased bacterial resistance is not clear, but it is known that all bacterial populations are heterogenous in their susceptibility to a given antibiotic.  The resistant bacteria may arise spontaneously as a result of mutation or adaptation.  In some occasions, resistance to an antibiotic is due to the production of specific enzyme (penicillinase produced by staphylococci) that destroys the drug penicillin.  It is also possible that an organism can be dependent on certain anitbiotics.

Improper use of anitbiotics can lead to serious consequences and decisive factor involved in the choice of proper antibiotic is the relative susceptibility of the invading microorganism.  In the laboratory, the in vitro determination of the susceptibility of an organism are accomplished in several ways:

1.  Test Tube Dilution Methods - a twofold serial dilutions of the antibiotics are prepared, and tubes are inoculated with suspension of test organism.  After a suitable incubation period at 35 C - 37 C, a measure of the susceptibility to the drugs is shown by the failure of the organism to grow.  Those organisms that show growth in both concentrations are reported as resistant.  The tube method is fairly accurate; however, it is a time consuming procedure, especially for lab techs working in a small hospital clinical lab and thus restricted to where quantitative results are of value.

2.  Diffusion Method - this method is simple and more rapid in determination.  The medium used can be blood agar plate (BAP); in the laboratory, we usually used Mueller-Hinton agar (MH).  The pure culture suspension, careful not to mix with other organisms, is streak over the surface of the plate by means of a sterile cotton swab.  Antibiotic discs are placed in equidistant manner using an antibiotic dispenser in order that zones of antibiotic inhibition can be read easily.  The plate is incubated overnight at a 35 C - 37 C.  The diameter of the zone of inhibition varies with the organism and antibiotic.  The results are strictly qualitative and is based upon the presence or absence or intermediate zone of inhibition.  Occasionally, isolated colonies can be seen growing within the zone of inhibition, and maybe resistant variants of the organism or a contaminant.

3.  Streak-plate Method - test organism is streaked with agar plate containing known values of antibiotics.  Inhibition of growth is noted.

4.  Automated Microbial System - efficient, accurate, rapid, and fast, clinical laboratories started using automated antimicrobial susceptibility testings in the late 70s and early 80s (MicroSystem, Vitek BioMereieux and other AMS brands).  Today, they are commonly used in many hospital labs.

4.  Susceptibility can also be found by chemical or physical changes - such as shift in pH, inhibition of hemolysis and reduction of hemoglobin.

Notes:

1.  Susceptibility tests are done when -

(a) a patient does not respond quickly to adequate therapy

(b) there is a relapse

(c) the diagnosis is unknown

(d) the disease is severe and fulminating.

2.  The size of the zone of inhibition depends on antibiotic solubility and diffusibility and does not necessarily coincide with bacterial activity.

3.  Important factors such as the following contribute to an accurate and reproducible report -

(a) the stability and solubility of antibiotics

(b) size of inoculum (important in tube method)

(c) composition of the culture medium

(d) the growth requirement of the organism, whether aerobic or anaerobic.

4.  Susceptibility tests maybe imperative for these organisms: Proteus, Pseudomonas aeruginosa, Staphylococcus aureus, coli-aerogenes group, Streptococcus (Enterococcus) faecalis; To organisms like Streptococcus pyogenes (GAS) and Neisseria meningitidis, susceptibility tests are not ordinarily done since they have not shown any tendency to change in resistance.

5.  The common antibiotics used for -

Gram positive isolates:

Penicillin (2 units discs are useful in unmasking colonies of coliform bacilli, Proteus species and Pseudomonas; 10 units discs are helpful in exposing colonies of Bordetella pertussis in Bordet-Gengou medium.  Penicillin is also the antimicrobial agent of choice for the treatment of pneumococcal infections.  Despite the widespread use of penicillin, there are no documented reports of penicillin-resistant pneumococci and no indication of a trend toward decreasing susceptibility. (This is important to research why and check if there are any recent documentation).  In patients who are allergic to penicillin, tetracycline and erythromycin have been considered as excellent alternative drugs for the treatment of pneumococcal infection. 

Ampicillin

Methicillin

Oxacillin

Cephalothin

Lincocin

Tetracycline

Erythromycin

Vancomycin

Gram negative isolates:

Tetracycline

Kanamycin (useful in separating Bacteroides and Clostridia species from other wound bacteria when incubated anaerobically).

Polymyxin B (use

Furadantin

Nalidixic acid

Neomycin (useful in exposing colonies of Group A Streptococci (GAS), enterococci and pneumococci)

Ampicillin (for Salmonella).

Chlorampenicol

The action of some of the antibiotics, like penicillin, is primarily bactericidal (killing of bacteria) while sulfanamides are bacteriostatic substances (inhibiting the growth of bacteria).  Abuses by physicians of prescribing antibiotics to patients suffering from cold (due to virus not bacteria) have contributed to resistant or multiple-resistant bacteria.  The fallacy of assuming that certain microbes will not develop resistance to certain antimicrobial agents should also be noted.

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Published  9/22/11  altgroup multiply
Web Page: ANTIBIOTICS