Antimicrobial agents dilution

The antimicrobial agent is diluted in the culture medium to a level at which it ceases to have any activity, for example phenols, cresols and alcohols (see Chapter 11). This method applies to substances with a high dilution coefficient, r. ... 

Antimicrobial agents with high r values are rapidly neutralized by dilution, while those with low T values are not good candidates for neutralization by dilution. 

Strictly, the definition should refer to substances that are antagonistic in dilute solution because it is necessary to exclude various common metabolic products such as alcohols and hydrogen peroxide. The term antibiotic is now commonly used for antimicrobial drugs in general, and it would be pedantic to object to this. Today, many commonly-used antibiotics are either fully synthetic or are produced by major chemical modification of naturally produced molecules hence, antimicrobial agent is perhaps a more accurate term, but antibiotic is much the commoner usage. 

Regional perfusion was first assessed in the equine literature as a technique for achieving high levels of antimicrobial agents in the carpus (Whitehair et al 1992). The basic principle involves the saturation of tissues with volume-diluted antimicrobial agent. The antimicrobial agent is... 

Intraperitoneal irrigation of antimicrobial agents for treatment of intraabdominal infection has been smdied often with conflicting results. Intraoperative antimicrobial irrigation does not improve patient outcomes in comparison with copious intraoperative irrigation with normal saline. Possibly the most important aspect of peritoneal irrigation is the dilutional effect on bacteria and adjuvants that promotes infection (intestinal contents and hemoglobin). Most system-ically administered antimicrobials easily cross the peritoneal membrane so that peritoneal fluid concentrations are similar to serum. " Confined areas, such as an abscess, can be expected to attain much lower antimicrobial concentrations. 

A number of nonspecific therapies have been advocated in the treatment and prevention of UTIs. Fluid hydration has been used to produce rapid dilution of bacteria and removal of infected urine by increased voiding. A critical factor appears to be the amount of residual volume remaining after voiding. As little as 10 mL of residual urine can alter the eradication of infection significantly. Paradoxically, increased dimesis also may promote susceptibifity to infection by diluting the normal antibacterial properties of the urine. Often in clinical practice the concentrations of antimicrobial agents in the mine are so high that dilution has little effect on efficacy. 

Kits NeoSpect and NeoTect contain no antimicrobial agents. Saline used for dilutions must be prepared without addition of any bacteriostatic agent (product monographs, Amersham Healthcare 2000 Berlex Laboratories 2001). 

Isotopic dilution is observed with higher concentrations of Tc in the first eluate therefore, only eluates from generators eluted regularly within 24 h after the previous elution may be used for labeling (Ponto et al. 1987). The GEA-Scan kit contains no antimicrobial agents (Immunomedics Europe 2000). 

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). 

One problem that may arise is when an antibiotic or preservative is the product, or part of the product under test. When this happens, the material must be inactivated or removed before sterility testing can take place. There are several methods of achieving this. Antibiotics, such as the penicillins, may be inactivated by the addition of the enzyme P-lactamase, whilst the action of sulphonamides can be blocked by the addition of />-aminobenzoic acid. Products containing preservatives or antimicrobial agents, such as benzoic acid, alcohols, or phenols, are diluted to the level at which the compound becomes ineffective. Products containing quaternary ammonium compounds can be inactivated by the addition of Tween, whilst many compounds containing heavy metals can be... 

The agar diffusion method (Kirby—Bauer) is also sometimes used for the evaluation of antibacterial activity of textiles. This is a relatively quick and easily executed semiquantitative method to determine antibacterial activity of diffusible antimicrobial agents on treated textile material. The bacteria are grown in nutrient broth medium and after appropriate dilution (e.g., lOOx) from the culture, test organisms are swabbed over the surface of agar plates. Ten-millimetre-diameter disks of the test fabric and control fabric are then gently pressed onto the surface of the plate. The plates are then incubated at 37 °C for 18—24 h. The antibacterial activity of the fabrics is demonstrated by the diameter of the zone of inhibition in comparison to the control textile sample. 

The determination of MIC of antibiotics was performed by microtiter method. Antimicrobial agents were diluted by serial dilutions MPB in flat-bottomed plates, they were also added to the culture of Staphylococcus aureus, and were incubated for 24 hr at 34°C. The control was culture Staphylococcus aureus without antimicrobial agents. After that mixture sowed on solid nutrient medium Mueller-Hinton agar for calculation of amount of colony forming particles and MIC definitions. The MIC was considered to be the lowest concentration, which retards the growth of Staphylococcus aureus during the incubation period. 

The lowest concentration of an antimicrobial agent that kills micro-organisms, as indicated by absence of growth following subculturing in the dilution method. 

If necessary the sample is dissolved and diluted in a suitable non-toxic diluent. Buffered sodium chloride-peptone solution to which an emulsifier and/or a neutraliser for antimicrobial agents may be added is widely used. 

Three methods may be used for the enumeration membrane filtration, plate count, and most probable number (MPN) method. The advantages of the membrane filter method are its low limit of detection (LOD) of < 1 CFU/g or mL and the efficient separation of the micro-organisms from components of the product, particularly antimicrobial agents. For the pour-plate method, the sample is generally 1 10 dissolved in the diluent, and 1 mL of the dilution is mixed with the agar. This corresponds to a LOD of 10 CFU/g or mL. The LOD is sometimes higher (e.g. 100 CFU/g or mL) if the product needs to be further diluted due to microbial inhibition, or lower in case of products with low microbial acceptance criteria. If the spread plate count technique is used the LOD is a factor of ten higher (>100 CFU/g or mL), because only 0.1 mL of the... 

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