OmpU appeared to be the dominant peak in an m/z range of 30,000 – 40,000 in the spectra of all 48 tested strains except for the spectrum representing the V. cholerae O1 strain of serotype Hikojima, where the most dominant peak was identified as OmpT. OmpU and OmpT are major outer membrane

proteins of V. cholerae [25]. OmpU is expressed Selleckchem SBE-��-CD when cells are colonizing a human host, while OmpT is repressed at this time [26]. Reproducible differences between the OmpU peak masses of different MLST genotypes ranging from 32.4 to 35.7 kDa enabled discrimination of epidemic isolates from less or non-pathogenic isolates. Sequencing of the ompU genes in V. cholerae isolates representing different genotypes and a database analysis revealed that the amino acid sequence of OmpU from the epidemic V. cholerae O1/O139 and O37 strains is highly conserved, while OmpU homologs from other V. cholerae isolates varied from this sequence. These differences in amino acid sequence resulted in almost all cases in mass differences of more than 70 Da, which was sufficient to distinguish the

“epidemic” OmpU proteins from OmpU proteins of other strains with the resolution of the method presented here. In general, differences in OmpU peak masses between strains were well reproducible in multiple experiments. However, small variations in the OmpU peak masses between separate experiments were observed, indicating that the method requires inclusion of a standard sample for calibration containing a characterized V. cholerae strain. Among the OmpU homologs of non-epidemic strains present LY411575 purchase in the NCBI database, one had a theoretical mass of 58 Da less than that of the “epidemic” OmpU protein, while in all other non-epidemic V. cholerae isolates the mass differed more than 70 Da. From the in silico analyzed 102 ‘epidemic’ isolates the theoretical mass of OmpU from eight, one and two isolates differed 58, 48 and 1 Da, respectively. Therefore, it can be assumed that epidemic strains (34,656 Da to 34,714 Da) can be distinguished from non-epidemic V. cholerae strains (less than 34,598 Da or more than 34,734 Da) based on OmpU using

the described MALDI-TOF Oxalosuccinic acid MS assay. The V. cholerae strain of serotype Hikojima was shown to produce both OmpU and OmpT (Figure 5). However, in the obtained MS-spectra OmpU was not detected well and therefore its peak mass was not determined. More isolates of the Hikojima serotype, which is a rare serotype, need to be tested to determine whether this result is strain or serotype specific [23]. The theoretical mass of OmpU of the tested strain is only one Da less than that of the N16961 OmpU. It should be noted that not all strains of serogroup O1 are toxigenic. Some strains are not able to produce the cholera toxin because these isolates lack the ctxAB and tcpA genes necessary for full virulence of V. cholerae [21, 27]. Furthermore, the Selleckchem Defactinib non-toxigenic O1 isolates in this study were also genetically distinct from the epidemic V.