Abstract: 9250 A. Introduction

1. General Discussion

Actinomycetes are a large, diverse group of Gram-positive bacteria in the phylum Actinobacteria. The order Actinomycetales includes facultative, anaerobic, filamentous, and Gram-positive bacteria that reproduce via fission or production of asexual spores or conidia. Their optimal environment has a pH between 7 and 8 and temperature between 15 and 20 °C.

Actinomycetes, such as Streptomyces and Nocardia spp., are widely distributed in nature. They may constitute a considerable proportion of the microbial population of soil and activated sludge. They may be found in lesser numbers in water environments (lake, river, and marine waters). Actinomycetes are considered important in antibiotic production and often are used in biodegradation studies.

Some actinomycetes are considered nuisance microorganisms in the water management industry because they may emit earthy or musty odors that affect the quality and public acceptance of municipal water supplies in many parts of the world. These bacteria are part of the normal flora of sand filters, where they can decompose organic compounds that are otherwise difficult to biodegrade. Actinomycete spores, especially Micromonospora spp., tolerate chlorine disinfection so well that a considerable fraction persists.¹

Water treatment plant operators find these naturally occurring odors difficult to remove via conventional treatment. As early as 1929, it was assumed that these odors could be attributed to volatile metabolites formed during normal actinomycete development.² Two such compounds, geosmin and 2-methyl-isoborneol, have been isolated^3–9 and identified as the agents responsible for earthy, musty odors in surface water and that impart an unpleasant taste to water.^9–11 However, both compounds are produced also by some filamentous cyanobacteria.^12–18 Geosmin and 2-methyl­isoborneol have threshold odor concentrations well below the microgram-per-liter level. One study found two actinomycete isolates that also produce 2,4,6- trichloranisole from the disinfection byproduct 2,4,6-trichloro-phenol, which creates a disagreeable odor.¹⁸ Traces of these products can impart a disagreeable odor to water or a muddy flavor to fish.

In areas periodically plagued by taste and odor compounds, it is prudent to enumerate actinomycetes. Identifying their relative abundance in a drinking water source or distribution system (e.g., in biofilms) can provide yet another means to assess water quality.¹⁹ The methods described in this section are well-established techniques that have been used successfully to isolate and enumerate actinomycetes related to public water supplies.^20–21 Actinomycetes, especially Nocardia spp. and Gordonia amarae, also have been recognized as a cause of disruptions in wastewater treatment. Massive growths can produce thick foam in the activated sludge process.^22–23

In terms of human health significance, some Nocardia spp. may be pathogenic,^24–25 whereas Streptomyces spp. are generally considered nonpathogenic.