Ruth Gordon addresses the absence of a clear, practical definition for microbial species in bacterial taxonomy. She argues that while the International Code of Nomenclature of Bacteria acknowledges that species definitions vary by individual opinion, this approach is inadequate for the fundamental classification unit. Gordon explains that microorganisms form spectrum-like continua rather than distinct groups, creating challenges when classifying based on limited strain samples. She illustrates this with Nocardia asteroides, where 138 strains showed variable growth at 45°C and acid production from rhamnose. Gordon proposes that a species definition should encompass populations of strains from diverse sources, including newly isolated strains, old stock strains, and their variants, united by correlated stable properties distinguishing them from other groups. She emphasizes studying many strains to identify truly stable characteristics and to prevent artificial species splitting. The definition accommodates natural variation while maintaining species coherence through pattern recognition of shared properties rather than requiring uniformity in all traits.

Key findings

• Microbial species lack an official, universally accepted definition despite being the basic classification unit in bacterial taxonomy.
• Bacteria exhibit spectrum-like relationships with variable characteristics; examining few strains can lead to artificial species divisions.
• A robust species definition should include freshly isolated strains, old stock strains, and their variants to identify the most stable, correlating properties.
• Species identity must be inherent in strain properties themselves, not dependent on isolation source or nomenclatural labels.
• The proposed definition requires studying many strains from multiple sources to reliably characterize species, accounting for natural microbial variation.