Sero types, phage types and antibiotic susceptibilities of Salmonella strains isolated from horses in The Netherlands from 1993 to 2000.

The research article studies the characteristics and antibiotic responses of Salmonella strains gathered from horses in Netherlands from 1993 to 2000. The researchers sought to understand the behaviour of different types of Salmonella strains in response to a variety of antibiotics, as well as observe any changes in susceptibility over time.

Methodology and Findings

• The research team studied 232 Salmonella strains isolated from horses manifesting symptoms of salmonellosis. These strains were gathered over a span of seven years, from 1993 to 2000.
• The strains were put through susceptibility tests against seven antimicrobial agents via the agar diffusion method. The process involves using agar as a medium to see how much bacteria can grow when exposed to antibiotics.
• Moreover, the samples were serotyped (classified based on their surface proteins) and phage typed for more specific variants of Salmonella enterica, namely subspecies enterica Typhimurium and Enteritidis.
• Two particular phage types of S. Typhimurium, PT 506 and 401 (both falling under DT 104 in the English phage typing system), were further put through susceptibility tests with chloramphenicol, streptomycin, and sulfonamides.
• Results showed that resistance was common against tetracycline and ampicillin. Meanwhile, most strains were susceptible to enrofloxacin and ceftiofur.
• The strains’ resistance to four antibiotics (tetracycline, kanamycin, ampicillin, and trimethoprim/sulfonamide combinations) noticeably decreased within the seven-year period, but resistance to gentamicin, ceftiofur and enrofloxacin remained stable during the same timeframe.

Salmonella Serovars and Phage Types

• The principal serovar found in the strains was S. Typhimurium, which displayed higher rates of multi-drug resistance compared to other Salmonella serovars.
• A total of 16 different resistance patterns were identified in the Salmonella strains. The most common patterns were resistance to tetracycline alone, and a compound resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline (symbolised as ACSSuT), along with resistance to ampicillin, kanamycin, and a combination of trimethoprim and sulfonamide (denoted as AKSxtT).
• The most prevalent phage type was the multi-resistant S. Typhimurium phage type 506 (DT 104), which commonly presented the ACSSuT resistance pattern. The S. Typhimurium phage type 401 (DT 104) was also often detected, commonly with an ASSuT resistance pattern.
• Significantly, the most frequently found S. Typhimurium phage types from horses echoed those found in humans, pigs, and cattle during that same period in the Netherlands. This illustrates the zoonotic nature of Salmonella where it can be transmitted between animals and humans.