The prevalence of antimicrobial resistance reduces the effectiveness of antimicrobial drugs in preventing and treating infectious diseases caused by pathogenic organisms, such as bacteria, fungi, and viruses. Because of the burgeoning growth of microbes with antimicrobial-resistant traits, there is a dire need to identify and develop novel and effective antimicrobial agents to treat infections from antimicrobial-resistant strains. The marine environment is rich in ecological biodiversity and can be regarded as an untapped resource for prospecting novel bioactive compounds. Therefore, exploring the marine environment for antimicrobial agents plays a significant role in drug development and biomedical research. Several earlier scientific investigations have proven that bacterial diversity in the marine environment represents an emerging source of structurally unique and novel antimicrobial agents. There are several reports on marine bacterial secondary metabolites, and many are pharmacologically significant and have enormous promise for developing effective antimicrobial drugs to combat microbial infections in drug-resistant pathogens. In this review, we attempt to summarize published articles from the last twenty-five years (1996–2020) on antimicrobial secondary metabolites from marine bacteria evolved in marine environments, such as marine sediment, water, fauna, and flora.

As the development of molecular serotyping approaches is critical for Salmonella spp., which include >2,600 serovars, we performed an initial evaluation of the ability to identify Salmonella serovars using (i) different molecular subtyping methods and (ii) a newly implemented combined PCR- and sequencing-based approach that directly targets O- and H-antigen-encoding genes. Initial testing was performed using 46 isolates that represent the top 40 Salmonella serovars isolated from human and nonhuman sources, as reported by the U.S. Centers for Disease Control and Prevention and the World Health Organization. Multilocus sequence typing (MLST) was able to accurately predict the serovars for 42/46 isolates and showed the best ability to predict serovars among the subtyping methods tested. Pulsed-field gel electrophoresis (PFGE), ribotyping, and repetitive extragenic palindromic sequence-based PCR (rep-PCR) were able to accurately predict the serovars for 35/46, 34/46, and 30/46 isolates, respectively. Among the methods, S. enterica subsp. enterica serovars 4,5,12:i:−, Typhimurium, and Typhimurium var. 5− were frequently not classified correctly, which is consistent with their close phylogenetic relationship. To develop a PCR- and sequence-based serotyping approach, we integrated available data sources to implement a combination PCR-based O-antigen screening and sequencing of internal fliC and fljB fragments. This approach correctly identified the serovars for 42/46 isolates in the initial set representing the most common Salmonella serovars, as well as for 54/63 isolates representing less common Salmonella serovars. Our study not only indicates that different molecular approaches show the potential to allow for rapid serovar classification of Salmonella isolates, but it also provides data that can help with the selection of molecular serotyping methods to be used by different laboratories.

Salmonella is a diverse foodborne pathogen, which has more than 2600 recognized serovars. Classification of Salmonella isolates into serovars is essential for surveillance and epidemiological investigations; however, determination of Salmonella serovars, by traditional serotyping, has some important limitations (e.g. labor intensive, time consuming). To overcome these limitations, multiple methods have been investigated to develop molecular serotyping schemes. Currently, molecular methods to predict Salmonella serovars include (i) molecular subtyping methods (e.g. PFGE, MLST), (ii) classification using serovar-specific genomic markers and (iii) direct methods, which identify genes encoding antigens or biosynthesis of antigens used for serotyping. Here, we reviewed reported methodologies for Salmonella molecular serotyping and determined the "serovar-prediction accuracy", as the percentage of isolates for which the serovar was correctly classified by a given method. Serovar-prediction accuracy ranged from 0 to 100%, 51 to 100% and 33 to 100% for molecular subtyping, serovar-specific genomic markers and direct methods, respectively. Major limitations of available schemes are errors in predicting closely related serovars (e.g. Typhimurium and 4,5,12:i:-), and polyphyletic serovars (e.g. Newport, Saintpaul). The high diversity of Salmonella serovars represents a considerable challenge for molecular serotyping approaches. With the recent improvement in sequencing technologies, full genome sequencing could be developed into a promising molecular approach to serotype Salmonella.

The wide usage of antibiotics contributes to the increase in the prevalence of antibiotic-resistant Salmonella. Plasmids play a critical role in horizontal transfer of antibiotic resistance markers in Salmonella. This study aimed to screen and characterize plasmid profiles responsible for antibiotic resistance in Salmonella and ultimately to clarify the molecular mechanism of transferable plasmid-mediated antibiotic resistance. A total of 226 Salmonella isolates were examined for antimicrobial susceptibility by a disk diffusion method. Thirty-two isolates (14.2%) were resistant to at least one antibiotic. The presence of plasmid-mediated quinolone resistance (PMQR) genes and β-lactamase genes were established by PCR amplification. PCR-based replicon typing (PBRT) revealed that these 32 isolates represented seven plasmid incompatibility groups (IncP, HI2, A/C, FIIs, FIA, FIB, and I1), and the IncHI2 (59.4%) was predominant. Antibiotic resistance markers located on plasmids were identified through plasmid curing. Fifteen phenotypic variants were obtained with the curing efficiency of 46.9% (15/32). The cured plasmids mainly belong to the HI2 incompatibility group. The elimination of IncHI2 plasmids correlated with the loss of β-lactamase genes (blaOXA-1 and blaTEM-1) and PMQR genes (qnrA and aac(6’)-Ib-cr). Both IncHI2 and IncI1 plasmids in a S. enterica serovar Indiana isolate SJTUF 10584 were lost by curing. The blaCMY-2-carrying plasmid pS10584 from SJTUF 10584 was fully sequenced. Sequence analysis revealed that it possessed a plasmid scaffold typical for IncI1 plasmids with the unique genetic arrangement of IS1294-△ISEcp1-blaCMY-2-blc-sugE-△ecnR inserted into the colicin gene cia. These data suggested that IncHI2 was the major plasmid lineage contributing to the dissemination of antibiotic resistance in Salmonella and the activity of multiple mobile genetic elements may contribute to antibiotic resistance evolution and dissemination between different plasmid replicons.