The genus Vibrio, which belongs to the family Vibrionaceae and the class Gammaproteobacteria, includes many species that are potential human pathogens . V. cholerae is a highly diverse species that consists of more than 200 serogroups . Strains within the serogroups O1 and O139 produce cholera toxin and are the causative agents of endemic and epidemic cholera, which represent an important cause of morbidity and mortality in countries with inadequate access to clean water and sanitation facilities . V. cholerae strains not included in these serogroups as well as other Vibrio spp. are referred to as non-cholera Vibrio spp. and are ubiquitous aquatic bacteria with a worldwide distribution, especially in warm estuarine and marine ecosystems . These halophilic bacteria prefer low to moderate salinity (less than 25 parts per thousand (ppt) NaCl) . The abundance of Vibrio spp. in marine and estuarine waters closely corresponds with the sea surface temperatures (SSTs) since they proliferate in warm water . Thus, regional variations in environmental conditions are paramount importance in understanding the ecology of Vibrio spp.
Human infections with non-cholera Vibrio spp. can manifest as wound infections, ear infections, gastroenteritis, and primary septicaemia and have been predominantly reported in tropical and subtropical regions . In Europe, cases are rare, and infections associated with the Mediterranean Sea [7–9], the Atlantic Ocean [10–12], or the Baltic Sea [13,14] have only been sporadically reported. However, a rapidly warming marine environment accompanied by an increase in extreme weather events such as heatwaves has resulted in unprecedented peak SSTs favouring the spread of Vibrio spp. worldwide. Recently, larger Vibrio spp. outbreaks have been reported in temperate regions such as Spain , Sweden, and Finland . In Germany, Vibrio spp. other than toxigenic V. cholerae strains were not classified as reportable pathogenic agents before March 2020; to date, only a few cases have been reported and few case series of autochthonous infections have been published [17–20]. Over the last decades, resistance to various antibiotics, including to ampicillins, tetracyclines, and carbapenems, has emerged among Vibrio spp [21,22]. As a result of resistant isolates, the monitoring of antibiotic profiles of clinical and environmental Vibrio spp. has become of increasing importance. However, information on the number of human cases and the abundance of Vibrio spp. in coastal waters is scarce in most other European countries.
When we became aware of a high number of human infections with virulent Vibrio strains contracted in the Baltic Sea during the summer months of 2018 and 2019 (data not shown), we decided to conduct an observational retrospective multi-centre cohort study of domestically-acquired Vibrio infections. This study describes the epidemiology and the clinical impact of those infections and assesses antibiotic resistance patterns and phylogenetic relationships among clinical isolates. To determine the association between Vibrio infections and SST anomalies, we analysed the degree of warming in the south-western Baltic Sea, where the majority of the infections occurred.