VIII. METHODS FOR TYPING MYCOBACTERIA
3 63
Indeed, the frequency of dual infection may be greater than these studies
would suggest even in developed countries where tuberculosis is coming
under control.
Further data and comment regarding dual infection in man have been
supplied by Bates et al. (1975) who studied isolates of tubercle bacilli
obtained from patients who were infected in two or more different anatomic
sites. I n this study single colony isolates were not evaluated but instead
numerous colonies from each culture were mixed together and subcultured
for typing. This technical point is noteworthy because of the overshadowing effect of resistant bacteria in any study involving resistant and susceptible strains. Since Type A is resistant to all phages except MTPH 1 and
M T P H 2 (see Table I) if this strain is present together with bacilli of any
other type, the lytic pattern of the latter will not be apparent because the
potential zones of lysis will be overgrown by the Type A phage-resistant
bacilli present on the medium.
The point is obvious. If more than one phage type is present in a culture,
present typing methods will not detect the type which is more susceptible
to phage lysis. Despite this limitation, the results showed that two of the
88 patients with concurrent infection at two or more sites were infected
with tubercle bacilli of different phage type. Thus despite inadequate
techniques, dual infection was recognised with unexpected frequency
among patients in the United States. These reports suggest that repeated
exogenous infection with tubercle bacilli does occur in developed countries,
and it can be speculated that exogenous reinfection is much more frequent
among persons living in locations where tuberculosis continues in epidemic
proportions.
Efforts to phage type Mycobacterium bovis have been unproductive and
no satisfactory technique for recognition of different types within this
species has emerged. Baess (1969) studied 25 M. bovis strains for their
susceptibility to nine phages and found all strains resistant to lysis. Caroli
et al. (1967) reported the phage typing of 75 strains of M. bovis isolated
from cattle in Italy and observed seven phage types using 14 different
phages. Redmond (1973) adapted seven phages to lyse M. bovis and used
these adapted phages to type 53 cultures of bovine tubercle bacilli isolated
from cattle and other animals in widely separated areas of the United
States. Prior to adaptation, the phages were inactive for bovine mycobacteria. Ten types were observed with only three cultures being resistant
to all phages. T o date, no other investigations have been reported.
The epidemiology of M . kansasii infection is poorly understood. Clinical
evidence indicates that it is not transmitted from man to man and yet a
reservoir in nature has not been clearly identified. Bailey et al. (1970) and
Kaustow et al. (1974) reported isolation of M. kansasii from tap water, and
3 63
Indeed, the frequency of dual infection may be greater than these studies
would suggest even in developed countries where tuberculosis is coming
under control.
Further data and comment regarding dual infection in man have been
supplied by Bates et al. (1975) who studied isolates of tubercle bacilli
obtained from patients who were infected in two or more different anatomic
sites. I n this study single colony isolates were not evaluated but instead
numerous colonies from each culture were mixed together and subcultured
for typing. This technical point is noteworthy because of the overshadowing effect of resistant bacteria in any study involving resistant and susceptible strains. Since Type A is resistant to all phages except MTPH 1 and
M T P H 2 (see Table I) if this strain is present together with bacilli of any
other type, the lytic pattern of the latter will not be apparent because the
potential zones of lysis will be overgrown by the Type A phage-resistant
bacilli present on the medium.
The point is obvious. If more than one phage type is present in a culture,
present typing methods will not detect the type which is more susceptible
to phage lysis. Despite this limitation, the results showed that two of the
88 patients with concurrent infection at two or more sites were infected
with tubercle bacilli of different phage type. Thus despite inadequate
techniques, dual infection was recognised with unexpected frequency
among patients in the United States. These reports suggest that repeated
exogenous infection with tubercle bacilli does occur in developed countries,
and it can be speculated that exogenous reinfection is much more frequent
among persons living in locations where tuberculosis continues in epidemic
proportions.
Efforts to phage type Mycobacterium bovis have been unproductive and
no satisfactory technique for recognition of different types within this
species has emerged. Baess (1969) studied 25 M. bovis strains for their
susceptibility to nine phages and found all strains resistant to lysis. Caroli
et al. (1967) reported the phage typing of 75 strains of M. bovis isolated
from cattle in Italy and observed seven phage types using 14 different
phages. Redmond (1973) adapted seven phages to lyse M. bovis and used
these adapted phages to type 53 cultures of bovine tubercle bacilli isolated
from cattle and other animals in widely separated areas of the United
States. Prior to adaptation, the phages were inactive for bovine mycobacteria. Ten types were observed with only three cultures being resistant
to all phages. T o date, no other investigations have been reported.
The epidemiology of M . kansasii infection is poorly understood. Clinical
evidence indicates that it is not transmitted from man to man and yet a
reservoir in nature has not been clearly identified. Bailey et al. (1970) and
Kaustow et al. (1974) reported isolation of M. kansasii from tap water, and