AUTHOR=Maier Christopher , Huptas Christopher , von Neubeck Mario , Scherer Siegfried , Wenning Mareike , Lücking Genia 

TITLE=Genetic Organization of the aprX-lipA2 Operon Affects the Proteolytic Potential of Pseudomonas Species in Milk

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.01190

DOI=10.3389/fmicb.2020.01190

ISSN=1664-302X

ABSTRACT=<p>Psychrotolerant <italic>Pseudomonas</italic> species are a main cause of proteolytic spoilage of ultra-high temperature (UHT) milk products due to the secretion of the heat-resistant metallopeptidase AprX, which is encoded by the first gene of the <italic>aprX-lipA2</italic> operon. While the proteolytic property has been characterized for many different <italic>Pseudomonas</italic> isolates, the underlying <italic>aprX-lipA2</italic> gene organization was only described for a few strains so far. In this study, the phylogenomic analysis of 185 <italic>Pseudomonas</italic> type strains revealed that the presence of <italic>aprX</italic> is strongly associated to a monophylum composed of 81 species, of which 83% carried the <italic>aprX</italic> locus. Furthermore, almost all type strains of known milk-relevant species were shown to be members of the three monophyletic groups <italic>P. fluorescens</italic>, <italic>P. gessardii</italic>, and <italic>P. fragi</italic>. In total, 22 different types of <italic>aprX-lipA2</italic> genetic organizations were identified in the genus, whereby 31% of the species tested carried the type 1 operon structure consisting of eight genes (<italic>aprXIDEF prtAB lipA2</italic>). Other genetic structures differed from type 1 mainly in the presence and location of genes coding for two lipases (<italic>lipA1</italic> and <italic>lipA2</italic>) and putative autotransporters (<italic>prtA</italic> and <italic>prtB</italic>). The peptidase activity of 129 strains, as determined on skim milk agar and in UHT-milk, correlated largely with different <italic>aprX-lipA2</italic> gene compositions. Particularly, isolates harboring the type 1 operon were highly proteolytic, while strains with other operon types, especially ones lacking <italic>prtA</italic> and <italic>prtB</italic>, exhibited significantly lower peptidase activities. In conclusion, the phylogenomic position and the <italic>aprX-lipA2</italic> gene organization specify the proteolytic potential of <italic>Pseudomonas</italic> isolates. In addition, however, an interplay of several environmental factors and intrinsic traits influences production and activity of AprX, leading to strain-specific proteolytic phenotypes.</p>