Dissemination and characterization of Stenotrophomonas maltophilia isolates from Dairy Cows in Northeast China

Stenotrophomonas maltophilia is a non-fermenting Gram-negative opportunistic pathogen that could survive in almost any humid environment (Chang et al. 2015). It could cause infections, especially pneumonia and bacteremia in immunocompromised patients (Brooke et al. 2017). One of the difficulties in treating infected patients is the low susceptibility to most antimicrobial agents currently used in therapy (Kullar et al. 2022; Tamma et al. 2022; Maraolo et al. 2023). Unlike other clinically common resistant bacteria, this low sensitivity to antibiotics is not due to the selective pressure caused by antibiotic use under treatment (Rello et al. 2019). Indeed, S. maltophilia is intrinsically resistant to various antimicrobial agents, including β-lactams, carbapenems, aminoglycosides, and quinolones. Among these, the inherent β-lactam resistance is due to the production of two different β-lactamases-L1 (group 2e) and L2 (group 3c) (Alonso and Martínez 1997). The L1 enzymes belong to molecular class B3 Zn²⁺-dependent metallo-β-lactamases, which can hydrolyze most β-lactams, including cephalosporins, penicillins, and carbapenems. Besides, the bla_L1 and bla_L2 genes could exist alone or simultaneously on S. maltophilia chromosomes (Han et al. 2020). For its natural multiple drug resistance, therapy against infections with S. maltophilia is a significant challenge for clinicians and public health.

The transmission of S. maltophilia is mainly attributed to nosocomial routes, for the number of immunocompromised individuals is high in health-care settings (Berg et al. 2014; Li et al. 2019). However, S. maltophilia is known to be widely distributed in environments. It has been isolated from soils and plant roots, some animals, and river water (Paopradit et al. 2017). Previous studies have examined the relationship between clinical S. maltophilia isolates and environmental isolates (Paopradit et al. 2017; Kardan-Yamchi et al. 2021). Kardan-Yamchi et al. (2021) confirmed the clonal relatedness of S. maltophilia isolates from the clinic and environment, and intra-hospital dissemination of S. maltophilia was further demonstrated. However, the transmission of S. maltophilia between food animals has not yet been reported. Here, we investigated the genetic relationship among S. maltophilia isolates that originated from dairy cows distributed between two adjacent provinces of China and performed the drug resistance genes analysis.

The detailed sampling sites were Tongliao, Inner Mongolia, and Changchun, Jilin, in northeast China. The distance between them is less than 250 km as the straight line (Fig. 1). A set of 54 and 52 Tongliao and Changchun samples were collected, respectively. Altogether 24 strains were identified from enrichment culture as S. maltophilia isolates. In addition, four distinct strains were isolated from two samples in Tongliao. The distribution and sources of isolates are shown in Table I.

Fig. 1.

As the results of ResFinder shown in Table I, all strains harbored the only β-lactamase gene bla_L1. No bla_L2 gene was detected. According to a study from 2020, Han et al. (2020) examined the diversity of bla_L1 and bla_L2 genes of S. maltophilia isolates from animal production environments in China. The finding in this study indicates that bla_L1 gene is the dominant genotype in S. maltophilia isolates from cow farms in northeast China. Apart from β-lactamase gene, there were aminoglycoside resistance genes in S. maltophilia, including aac(6′)-Iz and aph(3′)-IIc and aph(3′)-IIc carried by all 24 strains. Only two isolates from Changchun carried aac(6′)-Iz. The high prevalence of aac(6′)-Iz and aph(3′)-IIc in this study is consistent with Han et al. (2020) results.

The phylogenetic analysis was performed based on core genomes to further clarify the genetic contexts of S. maltophilia between the two places. The maximum-likelihood phylogenetic tree indicated that the 24 isolates were divided into four clonal groups, A–D (Fig. 1), and each group contained isolates from both locations. Among them, group D was the primary lineage (45.8%), containing a set of two strains from Changchun and nine from Tongliao. Group C was the second largest lineage (37.5%), among which the strains from Changchun had a high genetic relationship. Both B and A groups included two strains, but the two strains in each group were isolated from different sources. In addition, all four groups consisted of numerous small clusters. The clustering of strains from the same source into different branches indicated the clonal diversity of S. maltophilia in these two regions. The high genetic diversity of S. maltophilia strains in this study is consistent with previous studies from Tehran and Iran (Bostanghadiri et al. 2019; Kardan-Yamchi et al. 2021). Most notably, strains from Tongliao and Changchun gathering into the common clonal clusters I and II imply the presence of a common source of S. maltophilia and dissemination between the two regions (Fig. 1). These two clusters demonstrated the clonal relatedness of the strains from these regions. They provided evidence of the potential transmission between cow farms in northeast China. Previous studies have confirmed intra-hospital dissemination of S. maltophilia between the clinic and the environment (Kardan-Yamchi et al. 2021). In 2018, Kim et al. (2018) verified the high levels of similarity of environmental S. maltophilia isolates to clinical isolates and provided evidence that WWTPs may be the source of S. maltophilia spread to community and clinical settings.

Additionally, Bavaro et al. (2020) raised that immunocompromised hosts were exposed to S. maltopilia, which should arouse our alarm. However, few studies have focused on transmitting S. maltophilia between cow farms. The environment is a rich source of S. maltophilia that has the potential role of transmission. Therefore, Therefore, detecting the genetic relationship of S. maltophilia isolated from environments is necessary. Despite the genetic relationship between environmental and human strains lacking in this study, we should still be vigilant as the S. maltophilia strains can spread through the food chains.

To our best knowledge, this study first investigated the dissemination and characterization of S. maltophilia isolates from dairy cows in northeast China. The clonal diversity and clonal relatedness of the region were noted. Moreover, this study proved that the bla_L1 gene is the dominant β-lactamase gene of S. maltophilia in northeast China. Further studies with S. maltophilia isolates from human and farm environments are needed, and the surveillance for S. maltophilia in cow farms is warranted.