Aims: To investigate the biofilm formation by 122 Salmonella spp. and 48 Listeria monocytogenes strains on a plastic surface. Methods: Quantification of biofilm formation was performed in brain heart infusion (BHI), trypcase soya broth (TSB), meat broth (MB) and 1/20 diluted trypcase soya broth (1/20-TSB) in plastic microtitre plates. Results: All tested Salmonella spp. and L. monocytogenes strains produced biofilm in a suitable medium. However, the quantities of biofilm produced by Salmonella spp. were greater than those produced by tested L. monocytogenes strains. The nutrient content of the medium significantly influenced the quantity of produced biofilm. Diluted TSB was the most effective in promoting biofilm production by Salmonella spp., followed by TSB, while the least quantity of biofilm was formed in BHI and MB. L. monocytogenes produced the highest quantities of biofilm in BHI, followed by TSA, then MB, and the least quantities of biofilm were produced in 1/20-TSB. Conclusions: Salmonella spp. produces more biofilm in nutrient-poor medium, while L. monocytogenes produce more biofilm in nutrient-rich medium.

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) was recognized in Europe and worldwide in the late 1990s. Within a decade, several genetically and geographically distinct CA-MRSA lineages carrying the small SCC mec type IV and V genetic elements and the Panton-Valentine leukocidin (PVL) emerged around the world. In Europe, the predominant CA-MRSA strain belongs to clonal complex 80 (CC80) and is resistant to kanamycin/amikacin and fusidic acid. CC80 was first reported in 1993 but was relatively rare until the late 1990s. It has since been identified throughout North Africa, the Middle East, and Europe, with recent sporadic reports in sub-Saharan Africa. While strongly associated with skin and soft tissue infections, it is rarely found among asymptomatic carriers. Methicillin-sensitive S. aureus (MSSA) CC80 strains are extremely rare except in sub-Saharan Africa. In the current study, we applied whole-genome sequencing to a global collection of both MSSA and MRSA CC80 isolates. Phylogenetic analyses strongly suggest that the European epidemic CA-MRSA lineage is derived from a PVL-positive MSSA ancestor from sub-Saharan Africa. Moreover, the tree topology suggests a single acquisition of both the SCC mec element and a plasmid encoding the fusidic acid resistance determinant. Four canonical SNPs distinguish the derived CA-MRSA lineage and include a nonsynonymous mutation in accessory gene regulator C ( agrC ). These changes were associated with a star-like expansion into Europe, the Middle East, and North Africa in the early 1990s, including multiple cases of cross-continent imports likely driven by human migrations. IMPORTANCE With increasing levels of CA-MRSA reported from most parts of the Western world, there is a great interest in understanding the origin and factors associated with the emergence of these epidemic lineages. To trace the origin, evolution, and dissemination pattern of the European CA-MRSA clone (CC80), we sequenced a global collection of strains of the S. aureus CC80 lineage. Our study determined that a single descendant of a PVL-positive methicillin-sensitive ancestor circulating in sub-Saharan Africa rose to become the dominant CA-MRSA clone in Europe, the Middle East, and North Africa. In the transition from a methicillin-susceptible lineage to a successful CA-MRSA clone, it simultaneously became resistant to fusidic acid, a widely used antibiotic for skin and soft tissue infections, thus demonstrating the importance of antibiotic selection in the success of this clone. This finding furthermore highlights the significance of horizontal gene acquisitions and underscores the combined importance of these factors for the success of CA-MRSA.

Twenty-seven colistin-resistant, carbapenemase-producing Klebsiella pneumoniae isolates were identified from hospitals in Serbia. All isolates were bla _CTX-M-15 positive; ST101, ST888, ST437, ST336, and ST307 were bla _OXA-48 positive; and ST340 was bla _NDM-1 positive. ST307 had an insertion, and ST336 had a premature stop codon in the mgrB gene. Amino acid substitutions were detected in PmrAB of isolates ST101, ST888, ST336, and ST307. The mcr-1 and mcr-2 were not detected. An increase in phoP , phoQ , and pmrK gene transcription was detected for all sequence types.

Klebsiella pneumoniae is a major cause of severe healthcare-associated infections and often shows MDR phenotypes. Carbapenem resistance is frequent, and colistin represents a key molecule to treat infections caused by such isolates. Here we evaluated the antimicrobial resistance (AMR) mechanisms and the genomic epidemiology of clinical K. pneumoniae isolates from Serbia. Consecutive non-replicate K. pneumoniae clinical isolates (n = 2,298) were collected from seven hospitals located in five Serbian cities and tested for carbapenem resistance by disk diffusion. Isolates resistant to at least one carbapenem (n = 426) were further tested for colistin resistance with Etest or Vitek2. Broth microdilution (BMD) was performed to confirm the colistin resistance phenotype, and colistin-resistant isolates (N = 45, 10.6%) were characterized by Vitek2 and whole genome sequencing. Three different clonal groups (CGs) were observed: CG101 (ST101, N = 38), CG258 (ST437, N = 4; ST340, N = 1; ST258, N = 1) and CG17 (ST336, N = 1). mcr genes, encoding for acquired colistin resistance, were not observed, while all the genomes presented mutations previously associated with colistin resistance. In particular, all strains had a mutated MgrB, with MgrB^C28S being the prevalent mutation and associated with ST101. Isolates belonging to ST101 harbored the carbapenemase OXA-48, which is generally encoded by an IncL/M plasmid that was no detected in our isolates. MinION sequencing was performed on a representative ST101 strain, and the obtained long reads were assembled together with the Illumina high quality reads to decipher the bla_OXA–48 genetic background. The bla_OXA–48 gene was located in a novel IncFIA-IncR hybrid plasmid, also containing the extended spectrum β-lactamase-encoding gene bla_CTX–M–15 and several other AMR genes. Non-ST101 isolates presented different MgrB alterations (C28S, C28Y, K2^∗, K3^∗, Q30^∗, adenine deletion leading to frameshift and premature termination, IS5-mediated inactivation) and expressed different carbapenemases: OXA-48 (ST437 and ST336), NDM-1 (ST437 and ST340) and KPC-2 (ST258). Our study reports the clonal expansion of the newly emerging ST101 clone in Serbia. This high-risk clone appears adept at acquiring resistance, and efforts should be made to contain the spread of such clone.