Antibiotics microbial susceptibility

Although dinical microbiology laboratories report microbial susceptibility test results as sensitive or resistant to a particular antibiotic, this is not an absolute predictor of clinical response. In a given patient s infection, variables such as absorption of the drug, its penetration to the site of infection, and its activity once there (influenced, for example, by protein binding, pH, concentration of oxygen, metabolic state of the pathogen, intracellular location and concentration of microbes) profoundly alter the likelihood that effective therapy will result. 

Another reason why antibiotics are not detected in soils in appreciable amounts, if at all, is that most of them are highly labile and are rapidly degraded. In some cases the breakdown is attributable to chemical instability caused by pH, but more often it is the result of biological attack. Since most naturally occurring compounds are subject to microbial attack it is likely that all antibiotics are susceptible. 

Control of microbial susceptibilities toward approved antibiotics and antimycotics... 

Rifampin is a semisynthetic derivative of rifamycin, an antibiotic produced by Streptomyces mediterranei. It is active in vitro against gram-positive and gram-negative cocci, some enteric bacteria, mycobacteria, and chlamydia. Susceptible organisms are inhibited by less than 1 mcg/mL. Resistant mutants are present in all microbial populations at... 

Many microbial infections and toxins are spread by biofilms. Biofilm formation occurs on virtually every surface, starting with the adhesion of planctonic cells or small dispersed biofilm fragments. Proliferation of the cells is accompanied by the expression of an extracellular polysaccharide-based matrix [6], The cells embedded in this matrix are well protected and up to 1000 times less susceptible to antibiotics [7], Once a biofilm is formed, it is extremely difficult to remove this contamination. Thus, all antimicrobial surfaces should prevent the primary attack [8], One class of antimicrobial surfaces prevents the primary attack by creating surfaces that are not sticky to microbial cells, i.e., they do not allow adhesion of these cells. The other major class of antimicrobial surfaces is based on the killing of approaching microbes (see Fig. 2). Interestingly, both approaches can be achieved either by permanent surface modifications or by releasing bioactive compounds. 

These passive obstacles to antibiotic action complicate treatment of infectious disease in the absence of highly accurate diagnostic tools to identify the infectious pathogen. Intrinsic resistance is problematic but is predictable and easily identihed in the preclinical drug discovery stages. As a result, the spectrum of susceptible microbial species is weh known before the compound enters the clinic. 

Over the last six decades, however, increased use of antimicrobial drugs, not only in human medicine, but in other areas, such as veterinary medicine, agriculture, and fish farming, has had an enormous impact on the microbial society. Nearly everywhere, the numbers of susceptible strains have reduced and resistant strains or variants have increased in numbers. It has been repeatedly reported that the susceptibility profile of bacteria in any human compartment, such as the skin, intestine, and respiratory tract, is very different from what it was in the pre-antibiotic era, and even 15 years ago. The same trend is reported from hospitals and homes. Multidrug resistance, that is resistance to several antimicrobial drugs, is commonly found in bacteria that cause infections as well as in commensal organisms. 

Resistance to Tetracyclines. The tetracyclines still provide inexpensive and effective treatment for several microbial infections, but the emergence of acquired resistance to this dass of antibiotic has limited their clinical usefulness. Studies to define the molecular basis of resistance are underway so that derivatives having improved antibacterial spectra and less susceptibility to bacterial resistance may be developed. Tetracyclines are antibiotics of choice for relatively few human infections encountered in daily clinical practice (104), largely as a result of the emergence of acquired tetracycline-resistance among dinically important bacteria (88,105,106). Acquired resistance occurs when resistant strains emerge from previously sensitive bacterial populations by acquisition of resistance genes which usually reside in plasmids and/or transposons (88,106,107). Furthermore, resistance determinants contained in transposons spread to, and become established in, diverse bacterial species (106). 

Ciprofloxacin is a fluoroquinolone antibiotic that interferes with microbial DNA synthesis. It is indicated in the treatment of infections of the lower respiratory tract, skin and skin structure, bones and joints, urinary tract gonorrhea, chancroid, and infectious diarrhea caused by susceptible strains of specific organisms typhoid fever uncomplicated cervical and urethral gonorrhea women with acute uncomplicated cystitis acute sinusitis nosocomial pneumonia chronic bacterial prostatitis complicated intra-abdominal infections reduction of incidence or progression of inhalational anthrax following exposure to aerosolized Bacillus anthracis. Cipro IV Used for empirical therapy for febrile neutropenic patients. 

Levofloxacin is a fluoroquinolone/ophthalmic/antibiotic that interferes with microbial DNA synthesis. It is indicated in the treatment of acute maxillary sinusitis, acute bacterial exacerbation of chronic bronchitis, nosocomial pneumonia, community-acquired pneumonia, skin and skin structure infections, chronic bacterial prostatitis, urinary tract infection (UTI), inhalational anthrax (postexposure), and acute pyelonephritis caused by susceptible strains of specific microorganisms. Ophthalmic use is for the treatment of conjunctivitis caused by susceptible strains of aerobic Gram-positive and aerobic Gram-negative microorganisms. 

TESTING FOR MICROBIAL SENSITIVITY TO ANTIMICROBIAL AGENTS Bacterial strains, even from the same species, may vary widely in sensitivity to antibiotics. Information about the antibiotic sensitivity of the infecting microorganism is important for appropriate drug selection. Various methods are used to assess susceptibility, including disk-diffusion, dilution test, and automated broth dilution. The results are either reported on a semi-quantitative scale i.e., resistant, intermediate, or susceptible) or in terms of the minimal inhibitory concentration (MIC). 

In a number of cases, microbial resistance is mediated by the production of bacterial enzymes that attack the antibiotic molecule, changing its structure to an inactive form. This can lead to a so-called inoculum effect, in which a susceptible antibiotic is apparently less potent when larger numbers of bacteria are present in the medium than when fewer cells are employed. The more bacteria that are present, the more antibiotic-destroying enzyme that is present, and the more antibiotic that is required to overcome this to achieve the desired response. An antibiotic that is not enzyme modified is comparatively free of inoculum effects. 

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