DIAGNOSTIC MEDICAL MICROBIOLOGY
LABORATORY AIDS IN THE SELECTION OF ANTIMICROBIAL THERAPY
The antimicrobial drug used initially in the treatment of an infection is chosen on the basis of clinical impression after the physician is convinced that an infection exists and has made a tentative etiologic diagnosis on clinical grounds. On the basis of this "best guess," a probable drug of choice can be selected . Before this drug is administered, specimens are obtained for laboratory isolation of the causative agent. The results of these examinations may necessitate selection of a different drug. The identification of certain microorganisms that are uniformly drug-susceptible eliminates the necessity for further testing and permits the selection of optimally effective drugs solely on the basis of experience.
The commonly performed disk diffusion susceptibility test must be used judiciously and interpreted with
restraint. In general, only one member of each major class of drugs is represented. For staphylococci, penicillin G, oxacillin, cefazolin, erythromycin, gentamicin, and vancomycin are used. F or gram-negative rods, ampicillin,cefazolin and second- and third-generation cephalosporins, piperacillin and other "antipseudomonal penicillins,"carbapenems, trimethoprim-sulfamethoxazole, fluoroquinolones, and the aminoglycosides (amikacin,tobramycin, gentamicin) are included. F or urinary tract infections with gram-negative rods, nitrofurantoin,quinolones, and trimethoprim may be added. The choice of drugs to be included in a routine susceptibility test battery should be based on the susceptibility patterns of isolates in the laboratory,the type of infection (community-acquired or nosocomial), the source of the infection, and cost efficacy analysis for the patient population.
The sizes of zones of growth inhibition vary with the molecular characteristics of different drugs. Thus, the zone size of one drug cannot be compared to the zone size of another drug acting on the same organism. However, for any one drug the zone size can be compared to a standard, provided that media, inoculum size, and other conditions are carefully regulated. This makes it possible to define for each drug a minimum diameter of inhibition zone that denotes "susceptibility" of an isolate by the disk diffusion technique. The disk test measures the ability of drugs to inhibit the growth of bacteria. The results correlate reasonably
well with therapeutic response in those disease processes where body defenses can frequently eliminate
infectious microorganisms. In a few types of human infections, the results of disk tests are of little assistance (and may be misleading) because a bactericidal drug effect is required for cure. Outstanding examples are infective endocarditis, acute osteomyelitis, and severe infections in a host whose antibacterial defenses are inadequate, eg, persons with neoplastic diseases that have been treated with radiation and anti neoplastic chemotherapy, or persons who are being given corticosteroids in high dosage and are immunosuppressed.
Instead of the disk test, a semi-quantitative minimum inhibitory concentration (MIC) test procedure can be
used. It measures more exactly the concentration of an antibiotic necessary to inhibit growth of a standardized inoculum under defined conditions. A semi-automated microdilution method is used in which defined amounts of drug are dissolved in a measured small volume of broth and inoculated with a
standardized number of microorganisms. The end point,or minimum inhibitory concentration, is considered the last broth cup (lowest concentration of drug) remaining clear , i.e, free from microbial growth.
The minimum inhibitory concentration provides a better estimate of the probable amount of drug necessary to inhibit growth in vivo and thus helps in gauging the dosage regimen necessary for the patient.
Clinical microbiology laboratories perform disk diffusion tests and tests based upon determining the MIC and
interpret their results using guidelines established by the Clinical Laboratory and Standards Institute (CLSI)
located in Wayne, Pennsylvania. In addition, to help guide empiric therapy choices before the results of
antimicrobial susceptibility tests are available, it is recommended by CLSI that laboratories publish an
antibiogram annually that contains the results of susceptibility testing in aggregate for particular organism–drug combinations. For example, it may be important to know the most active -lactam antimicrobial agent targeted against Pseudomonas aeruginosa among ICU patients in a particular hospital so that agent can be used when a patient develops an infection while in that unit.There are other methods for assessing the efficacy of antimicrobial treatment. Bactericidal effects can be estimated by subculturing the clear broth onto antibiotic-free solid media. The result, eg, a reduction of colony forming units by 99.9% below that of the control, is called the minimal bactericidal concentration (MBC). The selection of a bactericidal drug or drug combination for each patient can be guided by specialized laboratory tests. Such tests measure either the rate of killing (time-kill assay) or the proportion of the microbial population that is killed in a fixed time (serum bactericidal testing). In urinary tract infections, the antibacterial activity of urine is far more important than that of serum.
CITED BY ANIL BHUJEL
Bsc Microbiology, TU.
Pokhara Bigyan Tatha Prabhidi Campus, Nayabazzar-9, Pokhara.
SELECTED REFERENCES URLS:
www.ncbi.nlm.nih.gov › NCBI › Literature › PubMed Central (PMC)