Clinical, molecular, and resistance features of Listeria monocytogenes in non-perinatal patients with listeriosis: 8-year retrospective data from four tertiary hospitals in Shandong, China

Introduction

Listeriosis, caused by the facultative anaerobic Listeria monocytogenes (L. monocytogenes), is a severe foodborne infectious disease, commonly resulting in high hospitalization and fatality rates approaching 50% (de Noordhout et al., 2014). Invasive infections can cause a series of severe clinical outcomes (e.g., meningitis, bacteremia, and meningoencephalitis) and are even life-threatening, particularly in the elderly or immunocompromised hosts (Schlech, 2019). Although listeriosis usually occurs infrequently and sporadically, outbreaks are reported occasionally (Papic et al., 2020). Because of its low incidence, the clinical evidence of human listeriosis is limited (Koopmans et al., 2023), without a comprehensive analysis of both epidemiological and microbiological data. In addition, the excessive use of antibiotics leads to the drug resistance of the pathogen L. monocytogene, aggravating the health burden and the concerns about the management of this disease (Maćkiw et al., 2016). Therefore, the investigation of listeriosis and pathogens is of great importance to develop appropriate prevention and disease management strategies (Pettinati, Telles & de Carvalho Ballian, 2006).

Over the past decades, the incidence of listeriosis has been increasing worldwide (Corral et al., 2024; Vazquez et al., 2024; Zahid et al., 2023). However, epidemiological surveillance of listeriosis was not widely initiated in China until 2013 (Lu et al., 2021). Accordingly, the limited reports on Chinese cases mostly come from regional and historical data, leading to the underrepresentation of the overall Chinese population (Feng et al., 2013; Zhang et al., 2019). The available data on molecular features, such as serotypes, virulence genes, and multilocus sequence typing (MLST), is even more limited (Fan et al., 2019). Thus, no specific diagnosis and treatment consensus for this disease has been established in China up to now (Lu et al., 2021); and meanwhile, the impact of demographic and clinical characteristics on prognosis has not been fully assessed. Within this context, urgent efforts are needed to gather more data to better understand the factors affecting treatment and prognosis. Besides, previous reports have demonstrated that L. monocytogenes has a high strain heterogeneity in terms of virulence potential and environmental adaption (Lakicevic, Den Besten & De Biase, 2021), which underscores the necessity of additional analysis in other regions to fill the missing data in this large population.

Herein, this study represented the first analysis of the clinical features, molecular characteristics, and antimicrobial resistance profiles of L. monocytogenes isolates in non-perinatal patients with listeriosis over 8 years in Shandong Province, China, aiming to provide additional evidence regarding the heterogeneity of this species with geographic variability.

Materials and Methods

Bacterial isolates and identification

Bacterial identification of isolates was performed based on the standard microbiological culture. After the collection and grind of clinical specimens, specimens were inoculated in a Columbia blood agar plate and stained with Gram stain. The CAMP testing was performed with a β-hemolytic strain of Staphylococcus aureus (ATCC 25923) and Streptococcus agalactiae. According to the morphological characteristics, Gram-positive, facultative anaerobes, Christie Atkins Munch-Peterson (CAMP)-positive, translucent colonies that produced a narrow β-hemolysis were presumptively identified as L. monocytogenes (Figs. 1A–1D). Further identification was confirmed by the automated Vitek-2 Compact system (bioMérieux, Lyons, France) with confidence levels of 99.9% for all strains and Vitek mass spectrometry system (bioMérieux, Lyons, France) with confidence levels of 98% (Figs. 1E, 1F).

Results

Incidence of listeriosis

Throughout the study period, a total of 27 cases with listeriosis were identified from 292,254 non-perinatal patients at four centers, with an overall incidence of nine cases per 100,000 admissions. The number of identified cases and annual incidence of listeriosis per 100,000 admissions over the study period is provided in Fig. 1. The annual incidence generally increased from 6.3 to 17.6 cases per 100,000 admissions between 2015 and 2019, and there was a large drop since 2020 (Fig. 2).

Discussion

This study, to the best of our knowledge, is the first to report the incidence of listeriosis in non-perinatal patients from Shandong, China and provide molecular and antimicrobial resistance profiles of L. monocytogenes isolates in this population. Our findings highlight the diversity and heterogeneity of this species, with various serogroups and sequence types. Additionally, the reports of treatment and outcomes of these patients and the resistance rate of L. monocytogenes isolates provide additional evidence for the management of listeriosis.

In our study, epidemiological data showed an overall incidence of nine cases per 100,000 admissions, which was higher than the historical data reported globally, ranging from 0.3/100,000 to 3/100,000 admissions (Corral et al., 2024; de Noordhout et al., 2014). This discrepancy may reflect unique factors in China, such as an ageing population and distinct dietary habits. In China, most cases of listeriosis are sporadic, with a similar distribution observed between perinatal and non-perinatal cases (Fan et al., 2019; Feng et al., 2013; Li et al., 2018). Four published reviews have previously reported the number and prevalence of listeriosis cases that occurred across different regions in China during various time periods: 147 cases from 1964 to 2010 (Feng et al., 2013), 253 cases from 2011 to 2016 (Li et al., 2018), 562 cases from 2011 to 2017 (Fan et al., 2019), and 759 cases from 2008 to 2017 (Chen et al., 2020). Similar to these reports, listeriosis cases were reported every year in our study, with a peak in specific years. According to the historical data before 2017, the prevalence peak of listeriosis occurred around 2014 (Chen et al., 2020; Fan et al., 2019); our recent data showed a similar tendency, in which the incidence increased between 2015 and 2019, followed by a sharp decline after 2020. Due to regional, period and population-specific differences, sustaining and comprehensive surveillance is urgently needed to understand listeriosis dynamics in China better. Nevertheless, this trend aligns with the pattern reported by Corral et al. (2024) during the same period, which may partly be attributed to the heightened public health measures during the COVID-19 pandemic, reducing the exposure to L. monocytogenes or to a shift in surveillance priorities toward monitoring COVID-19. Additionally, the overall incidence was unavailable in these reports due to the lack of comprehensive surveillance; thus, our study reported the data in the recent decade (2015–2022) and added a novel epidemiological perspective by providing annual incidence data. Overall, our findings may help change the current underestimation of this disease in China and highlight the necessity of strengthening epidemiological surveillance.

Because there was no established standard option and duration of therapy for listeriosis, patients herein received empirical therapies based on the clinical condition and clinicians’ experience. Consistent with previous practices (Corral et al., 2024; Sutter et al., 2024), β-lactam antibiotics (e.g., meropenem, ceftriaxone, and ampicillin) are commonly used for our cases. However, we did not observe a clear association of outcomes with treatment regimens or duration, possibly due to the small sample size and heterogeneity of clinical presentations. Despite limited cases, we observed that the majority of patients treated with penicillins were fully cured, which is in line with the international consensus on the priority of penicillin G or ampicillin for the treatment of listeriosis (Koopmans et al., 2023). While we used similar regimens, the lack of uniformity in treatment strategies might have contributed to the observed high mortality rate of 22.2%; nevertheless, this rate was comparable to the data in Asia (Chen et al., 2020; Choi et al., 2018; Fan et al., 2019) or Western populations (Dickstein et al., 2019; Jensen et al., 2016; Sutter et al., 2024), underscoring the challenges of treating listeriosis, even with timely and active antibiotic therapy. Our findings highlight the urgent need for further investigation of optimal or alternative therapeutic approaches to improve the clinical outcome and reduce the disease burden, as well as the development of evidence-based treatment guidelines tailored to the Chinese population.

The molecular characteristics observed from our non-perinatal cases are consistent with existing evidence in China (Cheng et al., 2022; Lu et al., 2021; Zhang et al., 2019), with serogroups 1/2a (66.7%) and 1/2b (25.9%) being most prevalent, and ST8 (44.4%) and ST87 (14.8%) as the predominant ST types. Moreover, the distributions of serogroups or MLST types were generally consistent among perinatal and non-perinatal patients, as demonstrated by a surveillance report in China (Li et al., 2019), supporting our findings. However, these findings were inconsistent with those in Western countries, where serotype 4b was frequently identified while ST87 was seldom linked to human listeriosis (Jennison et al., 2017). It is emphasized that regional surveillance is necessary for this heterogeneous species because specific STs are associated with severe listeriosis cases. Additionally, by reviewing the published literature, there is limited evidence about the virulence characterization of L. monocytogenes available, particularly in human listeriosis (Fan et al., 2019; Feng et al., 2013). In this study, plcA, prfA, inlA, inlJ genes were detected in almost all L. monocytogenes isolates, and inlB, iap, hly, and actA were detected in the majority of isolates. However, the association between virulence genes and pathophysiological mechanisms remain less well characterized and needs an in-depth study. Meanwhile, the favorable antibiotic susceptibility of L. monocytogenes isolates is reassuring here, with only two (7.4%) strains resistant to erythromycin or ciprofloxacin. These results are as expected on the basis of prior surveillance (Yan et al., 2019; Zhang et al., 2021).

Several limitations of our study should be acknowledged. Firstly, selection bias and data integrity due to insufficient records or loss of follow-up are inherent limitations of a retrospective study. Secondly, the small sample size and heterogeneity in included cases limit the generalizability of our findings and preclude a robust analysis of potential correlations between treatments and molecular features or susceptibilities. Thirdly, the data were collected from several regions in China, which cannot fully represent the national epidemiology of listeriosis. In addition, as our data were primarily derived from hospital-based clinical isolates, this may underestimate the prevalence of community-acquired cases and restrict a comprehensive analysis of environmental or foodborne sources. These limitations highlight the need for integrated surveillance systems combining public health, hospital, and food safety to understand listeriosis epidemiology comprehensively and support targeted prevention strategies.

Conclusion

In summary, this study provides valuable insights into the clinical and molecular features of L. monocytogenes in non-perinatal patients with listeriosis and adds new information regarding the heterogeneity within this species. Despite low antimicrobial resistance and appropriate treatment, listeriosis remains associated with a significant mortality rate, emphasizing the need for timely diagnosis and effective management strategies, especially in patients with comorbidities. The findings demonstrate that listeriosis remains a significant clinical concern, and molecular surveillance of L. monocytogenes strains is critical for understanding the epidemiology of the disease and guiding public health strategies, which may be a way forward in reducing the disease burden. Future studies should focus on larger patient cohorts and explore the potential predisposing factors of this infection.

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