Chapter 11
Chemical, Biological, and
Physical Methods
Most of the methods for detecting and characterizing microorganisms covered in this chapter have
been developed since 1960. Many can be used to estimate numbers of cells or quantity of cellular
byproducts. Unlike direct microscopic counts, most of those that follow are based on metabolic activity
of microorganisms on given substrates, measurements of growth response, measurements of some part
of cells including nucleic acids, or combinations of these.
CHEMICAL METHODS
The methods covered in this section are used primarily to detect, enumerate, or identify foodborne
organisms or their products:
Thermostable nuclease (for Staphylococcus aureus)
Limulus amoebocyte lysate (LAL) assay (for Gram-negative bacteria)
ATP assay (for live cells)
Radiometry
Fluorogenic/chromogenic substrates (to identify/differentiate microbial species or strains).
The relative sensitivity of these methods compared to others in this chapter can be seen from Table 11–1.
Thermostable Nuclease
The presence of S. aureus in significant numbers in a food can be determined by examining the
food for the presence of thermostable nuclease (DNase). This is possible because of the high correlation between the production of coagulase and thermostable nuclease by S. aureus strains, especially
enterotoxin producers. For example, in one study, 232 of 250 (93%) enterotoxigenic strains produced
coagulase, and 242 or 95% produced thermostable nuclease.
118 Non-S. aureus species that produce
DNAse are discussed in Chapter 23.
The examination of foods for this enzyme was first carried out by Chesbro and Auborn
32 employing
a spectrophotometric method for nuclease determination. They showed that as the numbers of cells
241
Chemical, Biological, and
Physical Methods
Most of the methods for detecting and characterizing microorganisms covered in this chapter have
been developed since 1960. Many can be used to estimate numbers of cells or quantity of cellular
byproducts. Unlike direct microscopic counts, most of those that follow are based on metabolic activity
of microorganisms on given substrates, measurements of growth response, measurements of some part
of cells including nucleic acids, or combinations of these.
CHEMICAL METHODS
The methods covered in this section are used primarily to detect, enumerate, or identify foodborne
organisms or their products:
Thermostable nuclease (for Staphylococcus aureus)
Limulus amoebocyte lysate (LAL) assay (for Gram-negative bacteria)
ATP assay (for live cells)
Radiometry
Fluorogenic/chromogenic substrates (to identify/differentiate microbial species or strains).
The relative sensitivity of these methods compared to others in this chapter can be seen from Table 11–1.
Thermostable Nuclease
The presence of S. aureus in significant numbers in a food can be determined by examining the
food for the presence of thermostable nuclease (DNase). This is possible because of the high correlation between the production of coagulase and thermostable nuclease by S. aureus strains, especially
enterotoxin producers. For example, in one study, 232 of 250 (93%) enterotoxigenic strains produced
coagulase, and 242 or 95% produced thermostable nuclease.
118 Non-S. aureus species that produce
DNAse are discussed in Chapter 23.
The examination of foods for this enzyme was first carried out by Chesbro and Auborn
32 employing
a spectrophotometric method for nuclease determination. They showed that as the numbers of cells
241