Quantitative antibiogram as a potential tool for epidemiological typing

Epidemiological typing of Alcaligenes xylosoxidans subsp. xylosoxidans by antibacterial susceptibility testing, fatty acid analysis, PAGE of whole-cell protein and pulsed-field gel electrophoresis

Antibacterial susceptibility testing, fatty acid analysis, protein analysis and DNA analysis of Alcaligenes xylosoxidans subsp. xylosoxidans were compared to determine the efficiency of the methods available for strain typing. Thirty isolates were investigated: 20 clinical isolates from a nonsocomial outbreak in Essen (Germany), 9 clinical isolates from sporadic nosocomial cases in Paris (France) and reference strain ATCC 2402. The highest microbiological discriminative power was exhibited by pulsed-field gel electrophoresis (PFGE) yielding nine types, followed by fatty acid methyl ester (FAME) analysis with six types, and antibacterial susceptibility testing and polyacrylamide gel electrophoresis with five types each. By combining the results of the four typing methods, 14 varieties could be differentiated. Protein analysis and fatty acid analysis failed to discriminate between isolates from Essen and Paris and the reference strain, while antibacterial susceptibility testing and DNA analysis clearly discriminated them. It is concluded that a combination of antibacterial susceptibility testing and PFGE typing is most suitable for epidemiological typing of Alcaligenes xylosoxidans subsp. xylosoxidans strains.

Multivariate analysis of antibiograms for typing Pseudomonas aeruginosa

A method for typing Pseudomonas aeruginosa using antibiotic susceptibility patterns is presented, which allows recognition of clusters of the same strain among clinical isolates from different patients, thus indicating whether cross infection has occurred. An index of similarity (the euclidean or the oblique distance), which includes all the differences of disk zone sizes among isolates, is computed and then elaborated by a clustering algorithm that successively groups all the isolates in larger clusters. The results of clustering are presented as dendrograms, whose terminal branches are pruned down to a level below which differences are casual; isolates that still appear on a common branch are considered identical. The reliability of this technique for detecting nosocomial cross infections was assessed by comparing its results with that of serotyping and pyocin typing. Only 2 of 31 (6.4%) clusters detected by multivariate analysis were not confirmed, while 4 of 33 (12.1%) clusters were recognized by serotyping and pyocin typing, but not by multivariate analysis. In at least two instances the differences in susceptibility patterns were due to cytoplasmic R factors. The routine use of antibiogram data for typing purposes should be considered an essential part of nosocomial infection control.

Cluster analysis of antibiotic susceptibility patterns of clinical isolates as a tool in nosocomial infection surveillance

Hospital infections represent a major epidemiological problem. The first step in the detection of nosocomial infections consists in assessing the probability that two or more isolates from different patients are similar or different. Many methods are available for typing purposes. Among these, antibiotic susceptibility patterns do not need extra cost or extra work and are available "on line" every moment they are needed. A mathematical technique of elaboration is proposed for disk zone sizes, in order to assess the probability of two or more clinical isolates to be the same strain. Antibiograms performed according to Kirby-Bauer are evaluated detecting zone sizes by a computer controlled device and then submitted to cluster analysis. Similarity of strains is reported in a dendrogram, in which strains are successively fused. Strains that share a common susceptibility pattern are considered a "cluster". At last, epidemiological maps are constructed for each group of strains, in which all the isolates are reported, ordered for patients, plotted on the day the specimen was collected, drawn in a different shape according to the source of specimen, and shadowed by the pattern of its cluster. This method of reporting data directly allows to detect cross infections among patients and can be used as a first typing step before other more expensive procedures.