Laboratory cross-contamination by Mycobacterium tuberculosis is known to be responsible for the misdiagnosis of tuberculosis, but its impact on other contexts has not been analyzed. We present the findings of a molecular epidemiology analysis in which the recent transmission events identified by a genotyping reference center were overestimated as a result of unnoticed laboratory cross-contamination in the original diagnostic laboratories.The phenomenon of misdiagnosing tuberculosis by laboratory cross-contamination when Mycobacterium tuberculosis is cultured has been widely reported (3, 4-8, 10, 11). The production of aerosolized particles after the processing of smearpositive specimens, cultures positive for M. tuberculosis, or positive control strains may be responsible for the inoculation of other specimens processed on the same day or of reagents used for the decontamination of specimens (5). False positivity is suspected (i) if M. tuberculosis is cultured from a sample processed together with a smear-positive specimen, (ii) if M. tuberculosis is cultured from only one of the cultures in the set (usually with a low yield of bacteria), and (iii) if the clinician is considering an alternative diagnosis, that is, a diagnosis other than tuberculosis (TB). Suspicion of false positivity is increased when two or more of these conditions are met. Finally, if molecular analysis is available, cross-contamination is confirmed when the strains cultured from both truly infected and contaminated specimens share the same genotypic pattern and no epidemiological links can be found between the cases. Several studies, some of which are based on molecular analysis, have estimated that the rate of laboratory cross-contamination for M. tuberculosis ranges from 0.1% to 3%, although massive contamination has caused up to 65% of false-positive cases (11).False-positive results for tuberculosis have been a matter of concern because of the clinical, therapeutic, and social impacts of the misdiagnosis of tuberculosis. The economic load associated with each misdiagnosed case of tuberculosis has been estimated to be $32,618 (9). However, another area on which false positivity has an impact but which has received little attention is the misidentification of recent transmission events by molecular epidemiology studies. Molecular epidemiology is based on the analysis of the genotypes of cultured M. tuberculosis isolates to identify cases infected by the same M. tuberculosis strain. These cases are defined as clustered and are considered to be caused by recent transmission events and to belong to the same transmission chain. If an analysis to determine the existence of potential false-positive cases is not performed before molecular analysis, as a quality control of microbiological procedures, there is a risk of misassigning clustered cases. This refined preanalysis is not usually performed because molecular epidemiology studies are generally run by laboratories which are different from those which culture M. tuberculosis from clinical spec...

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A total of 1,234 fecal samples from diarrhea cases were examined for etiological bacterial agents at medical facilities in La Paz and Sucre, Bolivia. Eighty strains of Shigella spp., 39 strains of Salmonella spp., 29 strains of Vibrio cholerae, and 222 strains of enteropathogenic Escherichia coli (139 EPEC, 55 ETEC, 29 EIEC, and 1 EHEC) were isolated. With regard to the serovars of Shigella, S. flexneri 2a, 3a, and lb were predominant. In the case of Salmonella, S. enteritidis was the most common, followed by S. typhi, S. poona, and S. paratyphi B. Out of 29 cholera strains, 25 belonged to biovar El Tor, serovar Ogawa while the remaining 4 were serovar Inaba. Among 55 strains of ETEC serotypes, 5 showed ST producers but none showed LT producers. Likewise, among 55 strains of enterohemorrhagic serotypes, only one strain (0157:H7) produced verocytotoxin (VT 2). The results of drug sensitivity tests revealed the predominance of Shigella, EPEC, and ETEC strains resistant to aminobenzil-penicillin (ABPC) and trimethoprim. Since diarrheal patients in Bolivia are treated mainly with ABPC or sulfamethoxazole/trimethoprim (SXT) and rarely with gentamicin, kanamycin, or other drugs, it is possible that ABPC-and SXT-resistant strains will increase and persist in the near future.

Molecular epidemiology of circulating clinical isolates is crucial to improve prevention strategies. The Spanish Working Group on multidrug resistant tuberculosis (MDR-TB) is a network that monitors the MDR-TB isolates in Spain since 1998. The aim of this study was to present the study of the MDR-TB and extensively drug-resistant tuberculosis (XDR-TB) patterns in Spain using the different recommended genotyping methods over time by a national coordinated system. Based on the proposed genotyping methods in the European Union until 2018, the preservation of one method, MIRU-VNTR, applied to selected clustered strains permitted to maintain our study open for 20 years. The distribution of demographic, clinical and epidemiological characteristics of clustered and non-clustered cases of MDR/XDR tuberculosis with proportion differences as assessed by Pearson’s chi-squared or Fisher’s exact test was compared. The differences in the quantitative variables using the Student's-t test and the Mann–Whitney U test were evaluated. The results obtained showed a total of 48.4% of the cases grouped in 77 clusters. Younger age groups, having a known TB case contact (10.2% vs 4.7%) and XDR-TB (16.5% vs 1.8%) were significantly associated with clustering. The largest cluster corresponded to a Mycobacterium bovis strain mainly spread during the nineties. A total of 68.4% of the clusters detected were distributed among the different Spanish regions and six clusters involving 104 cases were grouped in 17 and 18 years. Comparison of the genotypes obtained with those European genotypes included in The European Surveillance System (TESSy) showed that 87 cases had become part of 20 European clusters. The continuity of MDR strain genotyping in time has offered a widespread picture of the situation that allows better management of this public health problem. It also shows the advantage of maintaining one genotyping method over time, which allowed the comparison between ancient, present and future samples.