Lighting Up Mutation: a New Unbiased System for the Measurement of Microbial Mutation Rates

Mutators Enhance Adaptive Micro-Evolution in Pathogenic Microbes

Adaptation to the changing environmental conditions experienced within a host requires genetic diversity within a microbial population. Genetic diversity arises from mutations which occur due to DNA damage from exposure to exogenous environmental stresses or generated endogenously through respiration or DNA replication errors. As mutations can be deleterious, a delicate balance must be obtained between generating enough mutations for micro-evolution to occur while maintaining fitness and genomic integrity. Pathogenic microorganisms can actively modify their mutation rate to enhance adaptive micro-evolution by increasing expression of error-prone DNA polymerases or by mutating or decreasing expression of genes required for DNA repair. Strains which exhibit an elevated mutation rate are termed mutators. Mutators are found in varying prevalence in clinical populations where large-effect beneficial mutations enhance survival and are predominately caused by defects in the DNA mismatch repair (MMR) pathway. Mutators can facilitate the emergence of antibiotic resistance, allow phenotypic modifications to prevent recognition and destruction by the host immune system and enable switching to metabolic and cellular morphologies better able to survive in the given environment. This review will focus on recent advances in understanding the phenotypic and genotypic changes occurring in MMR mutators in both prokaryotic and eukaryotic pathogens.

The Quiet and Underappreciated Rise of Drug-Resistant Invasive Fungal Pathogens

Human fungal pathogens are attributable to a significant economic burden and mortality worldwide. Antifungal treatments, although limited in number, play a pivotal role in decreasing mortality and morbidities posed by invasive fungal infections (IFIs). However, the recent emergence of multidrug-resistant Candida auris and Candida glabrata and acquiring invasive infections due to azole-resistant C. parapsilosis, C. tropicalis, and Aspergillus spp. in azole-naïve patients pose a serious health threat considering the limited number of systemic antifungals available to treat IFIs. Although advancing for major fungal pathogens, the understanding of fungal attributes contributing to antifungal resistance is just emerging for several clinically important MDR fungal pathogens. Further complicating the matter are the distinct differences in antifungal resistance mechanisms among various fungal species in which one or more mechanisms may contribute to the resistance phenotype. In this review, we attempt to summarize the burden of antifungal resistance for selected non-albicansCandida and clinically important Aspergillus species together with their phylogenetic placement on the tree of life. Moreover, we highlight the different molecular mechanisms between antifungal tolerance and resistance, and comprehensively discuss the molecular mechanisms of antifungal resistance in a species level.

A spontaneous mutation in DNA polymerase POL3 during in vitro passaging causes a hypermutator phenotype in Cryptococcus species

Passaging of microbes in vitro can lead to the selection of microevolved derivatives with differing properties to their original parent strains. One well characterised instance is the phenotypic differences observed between the series of strains derived from the type strain of the human pathogenic fungus Cryptococcus neoformans. A second case was reported in the close relative Cryptococcus deneoformans, in which a well-studied isolate ATCC 24067 (52D) altered its phenotypic characteristics after in vitro passaging in different laboratories. One of these derivatives, ATCC 24067A, has decreased virulence and also exhibits a hypermutator phenotype, in which the mutation rate is increased compared to wild type. In this study, the molecular basis behind the changes in the lineage of ATCC 24067 was determined by next-generation sequencing of the parent and passaged strain genomes. This analysis resulted in the identification of a point mutation that causes a D270G amino acid substitution within the exonuclease proofreading domain of the DNA polymerase delta subunit encoded by POL3. Complementation with POL3 confirmed that this mutation is responsible for the hypermutator phenotype of this strain. Regeneration of the mutation in C. neoformans, to eliminate the additional mutations present in the ATCC 24067A genetic background, demonstrated that the hypermutator phenotype of the pol3^D270G mutant causes rapid microevolution in vitro but does not result in decreased virulence. These findings indicate that mutator strains can emerge in these pathogenic fungi without conferring a fitness cost, but the subsequent rapid accumulation of mutations can be deleterious.