The number of colony-forming units indicates what?

The concept of colony-forming units (CFUs) is crucial in microbiology, particularly in assessing bacterial populations. CFUs are used to estimate the number of viable bacteria or fungal cells in a sample. When a sample is inoculated onto a solid growth medium, each viable cell that can multiply and form a colony is counted and represented as a CFU. This method provides insight into the bacterial density present on a surface, allowing researchers and technologists to quantify contamination levels or bacterial load.

In contrast, the other options pertain to different aspects of microbiological analysis. The presence of viruses in a sample would require different techniques, such as polymerase chain reactions (PCR) or viral culture methods. Viability of bacterial spores is assessed through specific spore count techniques, not CFUs, since spores have different growth characteristics. The rate of bacterial metabolism involves measuring metabolic activity or growth rate rather than simply counting the number of colonies that form. Thus, CFUs primarily indicate the amount of bacteria present, making that answer accurate in this context.