Antimicrobial Susceptibility of Lactobacillus delbrueckii subsp. lactis from Milk Products and Other Habitats.

Noam Shani,Hélène Berthoud,Simone Oberhaensli,Hans-Peter Bachmann,Remo S Schmidt

doi:10.3390/foods10123145

Abstract

As components of many cheese starter cultures, strains of Lactobacillus delbrueckii subsp. lactis (LDL) must be tested for their antimicrobial susceptibility to avoid the potential horizontal transfer of antibiotic resistance (ABR) determinants in the human body or in the environment. To this end, a phenotypic test, as well as a screening for antibiotic resistance genes (ARGs) in genome sequences, is commonly performed. Historically, microbiological cutoffs (MCs), which are used to classify strains as either ‘sensitive’ or ‘resistant’ based on the minimal inhibitory concentrations (MICs) of a range of clinically-relevant antibiotics, have been defined for the whole group of the obligate homofermentative lactobacilli, which includes LDL among many other species. This often leads to inaccuracies in the appreciation of the ABR status of tested LDL strains and to false positive results. To define more accurate MCs for LDL, we analyzed the MIC profiles of strains originating from various habitats by using the broth microdilution method. These strains’ genomes were sequenced and used to complement our analysis involving a search for ARGs, as well as to assess the phylogenetic proximity between strains. Of LDL strains, 52.1% displayed MICs that were higher than the defined MCs for kanamycin, 9.9% for chloramphenicol, and 5.6% for tetracycline, but no ARG was conclusively detected. On the other hand, all strains displayed MICs below the defined MCs for ampicillin, gentamycin, erythromycin, and clindamycin. Considering our results, we propose the adaptation of the MCs for six of the tested clinically-relevant antibiotics to improve the accuracy of phenotypic antibiotic testing.

Highlights

The thermophilic lactic acid bacterium (LAB) Lactobacillus delbrueckii is important in many traditional fermented foods prepared worldwide (Table 1), and three subspecies have a long history of safe use [1]
The taxonomic assignment at the subspecies level was re-evaluated for three strains (Table 2): CIP 101810 and CIRM BIA 266 displayed a high similarity (98.27% and 98.61%, respectively) with the L. delbrueckii subsp. sunkii (LDS) type strain DSM 24966T, and CIRM BIA 1375 displayed similarity with L. delbrueckii subsp. lactis (LDL) DSM 20072T (98.53%)
Used alongside S. salivarius subsp. thermophilus, LDL is the major L. delbrueckii subspecies added as a thermophilic starter in the production of cooked Swiss cheese varieties

Summary

Introduction

The thermophilic lactic acid bacterium (LAB) Lactobacillus delbrueckii is important in many traditional fermented foods prepared worldwide (Table 1), and three subspecies (bulgaricus, delbrueckii, and lactis) have a long history of safe use [1]. LDB is mainly used in yogurt making, whereas LDL is traditionally used in the production of cooked cheeses, owing its tolerance to the high temperatures during the early phases of cheese manufacturing. Both subspecies seem to have developed a similar adaptation towards optimized utilization of milk resources through reductive evolution and limited acquisition of particular functions. Thermophilus and LDB, both of which grow in a mutualistic interaction by taking advantage of each other’s metabolism, has long been described [3].

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