No abstract

Salmonella Enteritidis causes infections in humans and animals which are often associated with extensive gut colonization and bacterial shedding in faeces. The natural presence of flagella in Salmonella enterica has been shown to be enough to induce proinflammatory responses in the gut, resulting in recruitment of polymorphonuclear cells, gut inflammation and, consequently, reducing the severity of systemic infection in chickens. On the other hand, the absence of flagellin in some Salmonella strains favours systemic infection as a result of the poor intestinal inflammatory responses elicited. The hypothesis that higher production of flagellin by certain Salmonella enterica strains could lead to an even more immunogenic and less pathogenic strain for chickens was here investigated. In the present study, a Salmonella Enteritidis mutant strain harbouring deletions in clpP and fliD genes (SE ΔclpPfliD), which lead to overexpression of flagellin, was generated, and its immunogenicity and pathogenicity were comparatively assessed to the wild type in chickens. Our results showed that SE ΔclpPfliD elicited more intense immune responses in the gut during early stages of infection than the wild type did, and that this correlated with earlier intestinal and systemic clearance of the bacterium.

Lactococcosis in fish has been associated with Lactococcus garvieae and the recently described L. petauri. However, the relevance of these emerging fish pathogens to Nile tilapia still requires thorough understanding. This study investigated lactococcosis outbreaks in Nile tilapia on Brazilian farms and characterized the isolates through molecular identification of the bacterial species, multilocus sequence typing (MLST) analysis, virulence to Nile tilapia, and antimicrobial susceptibility. Lactococcosis outbreaks were monitored from 2019 to 2022 throughout Brazil. The outbreaks occurred mainly during warmer months, and co-infections were observed in four farms, whereas concurrent bacterial infections were identified in all farms. Since the sequence of the 16S rRNA was not capable of differentiating between L. petauri and L. garvieae, Lactococcus spp. isolates were identified at the species level using the gyrB gene sequence. In total, 30 isolates were classified as L. petauri and two as L. garvieae. All L. petauri isolates were grouped in ST24, except for one isolate which belonged to the newly described ST47. A new ST was also described for the L. garvieae isolates identified, ST46. Furthermore, L. petauri ST24 and ST47 were characterized as singletons, whereas L. garvieae ST46 was grouped with ST16 and ST17 and formed CC17. For the challenge trial, an L. petauri ST24 isolate was chosen considering that this MLST lineage was the most frequently observed. L. petauri was reisolated from challenged Nile tilapia, confirming the pathogenicity of this bacterium to Nile tilapia. The infection in the fish progressed very rapidly, and within 48 h post-challenge clinical signs and the first mortalities were observed. The estimated LD50 was 5.74 x 103 CFU 15 days post-challenge. Provisional epidemiological cutoff values were determined for L. petauri for six antimicrobial agents from different drug classes. All isolates were characterized as wild type (WT) for neomycin and oxytetracycline, whereas 96.67 % of the isolates were characterized as WT for amoxicillin, erythromycin, and florfenicol, and 83.33 % were WT for norfloxacin. Of the 14 outbreaks analyzed, 12 were caused by L. petauri and two by L. garvieae. The gyrB gene sequence was used to differentiate L. petauri from L. garvieae and allowed for the correct identification of these pathogens. Two MSLT lineages of L. petauri were identified and ST24 was observed in different regions of the country, illustrating a rapid expansion of this bacterial lineage.