## 4 Discussion
Similar problems of non-specific and difficult to interpret results recorded here during the use of the former version, ISO 6579:2002/Amd 1:2007 (Annex D) have been previously reported [22], and show that false positives (e.g., _Citrobacter_, _Proteus_) increase the workload and costs of a control program. A previous comparison between XLD and BGA media showed that BGA is inferior to XLD due to its higher selectivity [42; 43], and the same was observed in the present study. Although the use of chromogenic media makes the recognition of most salmonellae easier, the present study showed that they fail in cases with rarer _Salmonella_ serovars. These serovars could come to be of epidemiological importance in later times if they are not accounted during control programs. Thus, failure of culture methods to recognize some _Salmonella_ serovars leads to a false assessment of the infectiousness of a given sample [40]. The suppression of competing bacteria in the original sample to allow the growth of _Salmonella_ to the level of detection is important, but it may also affect sensitivity of the conventional culturing media used. Sensitivity could be improved with the use of multiple culturing methods in parallel, but the combination increases workload and costs. Of interest was also the detection of lactose-positive _Salmonella_ isolates resembling _E. coli_ on XLD and SS agars. Lactose fermentation is a biochemical test differentiating _Salmonella_ from other Enterobacteriaceae. Since 1887, _E. coli_ has been recognized as a lactose fermenter and _Salmonella_ as a non-lactose fermenter. Therefore, this information was used for the development of the differential agar media XLD, SS, and BG. However, a few decades ago [PERSON] (1986) [44] reported that about 1% of all salmonellae ferment lactose, but the true incidence of lactose-fermenting (lac+) salmonellae is yet unknown, because most diagnostic laboratories do not further characterize lactose-positive colonies [45]. Lac+ _Salmonella_ strains of some important serovars, such as serovar Typhimurium, have been reported [46; 47; 48] from animal populations, and it is important for these to be accounted for during the application of control programs or during the investigation of suspect food-borne human cases. The types of colonies examined here would not have been selected for further testing if the ISO was strictly followed because they resembled typical lac+ _E. coli_. This would have resulted in their underreporting. False-negative samples may also occur if _Salmonella_ lac+ strains are masked by the growth of bacteria, such as _Klebsiella_ spp., which carry the lactose (lac) operon on a plasmid or a chromosome [49; 50]. Another important taxonomic characteristic of Enterobacteriaceae is the production of H\({}_{2}\)S. _Citrobacter_, _Proteus_, and _Salmonella_ are major H\({}_{2}\)S-producing genera. H\({}_{2}\)S production is, perhaps, helping the colonization of gut and, in synergy with other virulence determinants, the development of gastroenteritis [51]. About 97% of _Salmonella_ spp., with the exception of \(S\). Choleraeus and S. Typhi [52], produce H\({}_{2}\)S. However, this characteristic may be absent in environmental strains of _Salmonella_ spp. due to mutations [53; 54]. One pathway of H\({}_{2}\)S production is through the reduction of thiosulfate mediated by enzymethiosulfate reductase [51; 55; 56]. However, there are some problems in the interpretation of H\({}_{2}\)S production needing further consideration. Specifically:  
1. The content and availability of the sulfur source and peptones of the culture media. Peptones themselves contain varying amounts of sulfur amino acids and partially oxidized sulfur compounds, potentially giving a false positive result. 2. The commercial diagnostic media may also contain fermentable carbohydrates, resulting in the masking of H\({}_{2}\)S production by the production of acid during sugar fermentation either because it is repressed or because it cannot be detected. 3.