Assessing the virulence potential of different Listeria monocytogenes Clonal Complexes with Galleria mellonella larvae model

Introduction and Objective: Listeria monocytogenes is a pathogen causes infection mainly through food contamination. This species is well-known for its diversity, exhibiting differential virulence potential within strains from different clonal complexes (CCs). These can be defined as hypervirulent CCs, that are highly frequent among clinical cases and cause severe clinical outcomes (i.e. maternal-neonatal and central nervous system infections), or as hypovirulent CCs, that are associated with food and food processing environments and present attenuated virulence, causing disease in highly immunocompromised individuals. Many methodologies have been employed to study this differentiated pathogenicity, with Galleria mellonella larvae emerging as an in vivo infection model [1, 2]. The purpose of this study was to assess the pathogenic potential of L. monocytogenes isolates from eight distinct CCs, either hyper- and hypovirulent, through an in vivo infection and further analysis of its impact on the immune response of Galleria larvae. Methodology: Sixteen L. monocytogenes isolates from hypervirulent (CC1, CC2, CC4, CC6, CC87 and CC388) or hypovirulent (CC9 and CC121) CCs were selected for the purposes of this study. An infectious dose of 1x107 CFU/mL of L. monocytogenes was injected in the hindmost left proleg, into the hemocoel of the larvae. Results: The survival curves and the health index scores of the G. mellonella larvae infected with the 16 selected L. monocytogenes isolates are represented, respectively, in Figure 1A and 1B. The gathered results showed that: The two L. monocytogenes isolates that caused the highest and lowest survival rates are from hypervirulent CCs; Significant differences intra-clonal complex were observed within the CC6; Isolates from CC9 exhibited a hypervirulent phenotype when injected into G. mellonella larvae; At all time points, a decrease in the HISs was observed for larvae injected with each isolate. The observed HIS pattern was similar to that noted on the survival curves. Considering the pathogenic capacity of L. monocytogenes strains, a subset of five isolates was selected to pursue with further analysis of the host’s immune response based on the expression of the genes coding for antimicrobial peptides (AMPs). The obtained results showed an AMPs overexpression in strains that caused lower survival rates. Conclusions: The results showed that no correlation between hyper- or hypovirulent CCs and survival rates or HIS, as isolates causing higher and lower survival were from hypervirulent CCs. Additionally, the virulence potential of L. monocytogenes strains seems to be strain-dependent upon infection of G. mellonella larvae. Further studies need to be conducted with a larger sample size to draw more robust conclusions. Additionally, the expression of AMPs should be monitored throughout the infection process along with bacterial cells levels.