Pairwise Growth Dynamics of Environmental Escherichia coli and Enterococcus Isolates with Contrasting Antibiotic Susceptibility Phenotypes: Influence of Temperature

Background/Objectives: Environmental bacterial isolates with contrasting, previously established antimicrobial-susceptibility phenotypes may differ in growth under antibiotic-free conditions, but such differences can reflect strain background and temperature. This study characterized the monoculture population development and growth kinetics of predefined pairs of naturally occurring, genetically distinct environmental Escherichia coli, Enterococcus faecalis, and Enterococcus faecium isolates selected from a broader previously characterized collection. Methods: Nine isolates were grown separately in a controlled nutrient-enriched seawater model at 12, 22, and 30 °C, representing low, intermediate, and high water-temperature conditions relevant to the Thermaic Gulf. Growth was described using the Baranyi–Roberts model. Within each predefined strain pair and temperature, population development over time was assessed by two-way mixed-design ANOVA with Greenhouse–Geisser correction and Šídák-adjusted time-point comparisons. Temperature dependence of μmax was summarized using Ratkowsky and Arrhenius models. Results: Population-development patterns differed between paired isolates in all pair–temperature comparisons except MA60–T640 at 30 °C, but the direction and degree of the differences varied among pairs and sampling times. Fitted μmax did not always correspond directly to observed population development, as illustrated by SB10–T280. SB10 had a higher μmax at all three temperatures while remaining at lower absolute population levels during early-to-mid growth. Conclusions: The selected environmental isolate pairs did not show a uniform direction of difference associated with antimicrobial-susceptibility phenotype. Population-development patterns were pair- and time-specific and differed among the tested temperature conditions, while fitted μmax did not always reflect observed population development. These findings provide a temperature-resolved basis for interpreting the growth behaviour of selected environmental isolates with contrasting antimicrobial-susceptibility phenotypes under controlled nutrient-enriched marine conditions across temperatures relevant to warming shellfish-production environments.