Salmonella sp. serotyping is a very useful epidemiological tool to detect outbreaks in the developed world (5, 10). The Spanish National Salmonella Reference Laboratory routinely serotypes Salmonella sp. strains from most Spanish Autonomous Communities (AACC) for epidemiological surveillance (8, 9). In August of 1997 an atypical Salmonella enterica subsp. enterica serotype 4,5,12:i:− strain was detected. This strain either could be a monophasic strain of serotype Typhimurium (4,5,12:i:1,2) or serotype Lagos (4,5,12:i:1,5) or could represent a new serotype (7). The incidence of this serotype rapidly increased, and this serotype became the fourth most frequently encountered Salmonella sp. serotype in Spain during the second semester of 1997 (2.2% annual incidence). The aim of this study was to establish the incidence and possible origin of this serotype and to provide further characterization.
A total of 7,146 and 774 Salmonella sp. strains were isolated from human and food samples, respectively, in 1998 and the first quarter of 1999 in Spain. They were assigned to a serotype by the slide agglutination method with commercial antiserum (Sanofi Diagnostic Pasteur, Paris, France) and antiserum we had induced. Furthermore, the specific identification of phase 2 antigens of the H1 antigenic complex was performed by the multiplex PCR method of Echeita et al. (4). Antimicrobial resistance was determined by the disc diffusion method according to National Committee for Clinical Laboratory Standards criteria (6). The following antimicrobials were tested: ampicillin (A), chloramphenicol (C), sulfamethoxazole (Su), gentamicin (G), ciprofloxacin, kanamycin, streptomycin (S), cephalothin, nalidixic acid, cefotaxime, tetracycline (T), and co-trimoxazole (SxT). Phage susceptibility was studied by using a set of Salmonella serotype Typhimurium international phages (Laboratory of Enteric Pathogens, London, United Kingdom) according to the method of Anderson et al. (1).
Only Salmonella serotype 4,5,12:i:− strains which were detected by the slide agglutination method were selected for further studies. All selected strains were H:1,2 and H:1,5 negative by the multiplex PCR method. All of these strains were lysed only by the additional Salmonella serotype Typhimurium phage 10 (phage type [PT] U302). The most frequent resistance pattern was ACSuGSTSxT.
Two hundred and eighty-eight (3.7%) strains of Salmonella serotype 4,5,12:i:− with PT U302 and an ACSuGSTSxT multidrug resistance pattern were considered for the incidence study. Two hundred and seventy-one (3.8%) and 17 (2.2%) strains were isolated from human and food samples, respectively. Food strains were isolated from pork or pork products. Fourteen of 17 AACC were implicated. The distribution of these human strains was similar to that obtained when analyzing the total number of Salmonella spp. from humans in Spain for such characteristics as patient sex and age and date of isolation.
Salmonella serotype 4,5,12:i:− has been only sporadically isolated over the past few years in Spain (10 strains in the period 1993 to 1996) (3).
The annual incidence of this strain has increased during the period of this study, and this strain has become the fourth and ninth most common serotype isolated from humans and food, respectively. Now this serotype is affecting most Spanish AACC.
This report documents the first isolation of this strain with the multidrug resistance pattern described above. The resistance to gentamicin, a very active antimicrobial against most Salmonella spp. (2), is interesting. It can be concluded that an atypical, gentamicin-resistant Salmonella serotype 4,5,12:i:− strain has emerged and spread to humans in Spain, probably with contaminated pork as the source.
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