Living in an Extremely Polluted Environment: Clues from the Genome of Melanin-Producing Aeromonas salmonicida subsp. pectinolytica 34mel ^T

Abstract

d Aeromonas salmonicida subsp. pectinolytica 34mel T can be considered an extremophile due to the characteristics of the heavily polluted river from which it was isolated. While four subspecies of A. salmonicida are known fish pathogens, 34melT belongs to the only subspecies isolated solely from the environment. Genome analysis revealed a high metabolic versatility, the capability to cope with diverse stress agents, and the lack of several virulence factors found in pathogenic Aeromonas. The most relevant phenotypic characteristics of 34mel T are pectin degradation, a distinctive trait of A. salmonicida subsp. pectinolytica, and melanin production. Genes coding for three pectate lyases were detected in a cluster, unique to this microorganism, that contains all genes needed for pectin degradation. Melanin synthesis in 34melT is hypothesized to occur through the homogentisate pathway, as no tyrosinases or laccases were detected and the homogentisate 1,2-dioxygenase gene is inactivated by a transposon insertion, leading to the accumulation of the melanin precursor homogentisate. Comparative genome analysis of other melanogenic Aeromonas strains revealed that this gene was inactivated by transposon insertions or point mutations, indicating that melanin biosynthesis in Aeromonas occurs through the homogentisate pathway. Horizontal gene transfer could have contributed to the adaptation of 34mel T to a highly polluted environment, as 13 genomic islands were identified in its genome, some of them containing genes coding for fitness-related traits. Heavy metal resistance genes were also found, along with others associated with oxidative and nitrosative stresses. These characteristics, together with melanin production and the ability to use different substrates, may explain the ability of this microorganism to live in an extremely polluted environment.A eromonas salmonicida subsp. pectinolytica 34melT is a melanin-producing Aeromonas strain that degrades polypectate, which is an unusual characteristic among Aeromonas species (1). It was isolated from the heavily polluted water of the Riachuelo, the last part of the Matanza River, located in Buenos Aires, Argentina. This river has received the effluents of hundreds of tanneries and other industries, as well as urban sewage and fuels, for more than a century. Among the contaminants found in this environment, which has high organic matter and low dissolved oxygen contents, are hydrocarbons, polychlorinated biphenyls, pesticides, arsenic, and heavy metals, such as chromium, lead, copper, mercury, and nickel (2-4). A molecular analysis of the microbial diversity of river water and sediments from the isolation site showed the presence of bacteria belonging to several taxa. Analysis of 16S rRNA gene sequences revealed the presence of bacteria belonging to the Beta-, Gamma-, and Epsilonproteobacteria in the water, as well as a higher level of diversity in the sediments, in which Alpha-, Beta-, Gamma-, and Deltaproteobacteria, Firmicutes, and Bacteroidetes were detected (5).The ge...

Genomic and phenotypic characterization of an atypical Aeromonas salmonicida strain isolated from a lumpfish and producing unusual granular structures

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“…In the past, this characteristic has been used as a taxonomic marker to differentiate between Aeromonas species (Martin‐Carnahan & W. Joseph, ), but recent results challenge this assumption. The subspecies pectinolytica of the salmonicida species has the capacity to grow at 37°C (Pavan, Abbott, Zorzópulos, & Janda, ), as opposed to the other four officially recognized subspecies ( salmonicida, smithia, achromogenes and masoucida ), which are psychrophilic (Martin‐Carnahan & W. Joseph, ; Pavan, Pavan, López, Levin, & Pettinari, ). Furthermore, other putative A. salmonicida subspecies were recently reported to have the ability to grow at 37°C, confirming that pectinolytica is not an exception, while hinting at a greater phenotypic diversity in A. salmonicida than previously estimated and shedding light on an intraspecies lifestyle dichotomy (Vincent et al., ).…”

4-Hydroxyphenylpyruvate Dioxygenase Thermolability Is Responsible for Temperature-Dependent Melanogenesis in Aeromonas salmonicida subsp. salmonicida

“…It has been generally reported that the inactivation of HmgA, an enzyme that degrades HGA into maleylacetoacetate, results in pyomelanin production in many species, including mesophilic A. salmonicida subsp. pectinolytica (50)(51)(52)(53). By examining the hmgA genes of A. salmonicida subsp.…”

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