5.1 During the past decade, it has become increasingly apparent that non-tuberculous mycobacteria are common members of the indigenous MWF bacterial population. Measurement of mycobacterial cell count densities is an important step in establishing a possible relationship between mycobacteria and occupational health-related allergic responses, for example, hypersensitivity pneumonitis (HP) in persons exposed to aerosols of metalworking fluids. It is known that the viable mycobacteria count underestimates the total mycobacterial levels by not counting the non-culturable, possibly dead or moribund population that is potentially equally important in the investigation of occupational health-related problems. The direct microscopic counting method (DMC) described here gives a quantitative assessment of the total numbers of acid-fast bacilli. It involves using acid-fast staining to selectively identify mycobacteria from other bacteria, followed by enumeration or direct microscopic counting of a known volume over a known area. Although other microbes-particularly the Actinomycetes-also stain acid-fast, they are differentiated from the mycobacteria because of their morphology and size. Non-mycobacteria, acid-fast microbes are 50 to 100 times larger than mycobacteria. This practice provides quantitative information on the total (culturable and non-culturable viable, and non-viable) mycobacteria populations. The results are expressed quantitatively as mycobacteria per mL of metalworking fluid sample.

5.2 The DMC method using the acid-fast staining technique is a semi-quantitative method with a relatively fast turnaround time.

5.3 The DMC method can also be employed in field survey studies to characterize the changes in total mycobacteria densities of metalworking fluid systems over a long period of time.

5.4 The sensitivity detection limit of the DMC method depends on the MF and the sample volume (direct or centrifuged, etc.) examined.