Abstract
Viable bacteria were recovered from estuarine waters passed through a 0.2-mum polycarbonate membrane filter. The recovery method included the use of a dilute nutrient broth for primary enrichment followed by conditioning of the organism to a dilute nutrient solid medium. These bacteria were gram-negative rods and coccobacilli having an NaCl requirement and, upon initial culturing, low nutritional requirements. In response to increased nutrient preparations, these microorganisms underwent an increase in size and growth rate, giving rise to visible colonies. Phenotypic characterization suggests that species of Vibrio, Aeromonas, Pseudomonas, and Alcaligenes were among the isolates. The abundance and the nutritional requirements of these ultramicrobacteria imply that they represent a class of microorganisms which have successfully adjusted to poor nutrient conditions.