250
Number of Samples Revealing Aer. salmonicida
15/100 gill samples revealed the pathogen at 6.3 × 10
2 –1 × 10
4 /g
19/100 mucus samples revealed the pathogen at 9.1 × 10
2 –1.7 × 10
4 /g
Using CBB, Cipriano et al. ( 1996b ) reported Aer. salmonicida in higher numbers
from the mucus than kidney of six salmon. Thus, the populations of Aer. salmonicida were in the range of 1.1 × 10
3 to 1.8 × 10
7 /g and 1.0 × 10
3 to 1.4 × 10
7 /g for
mucus and kidney, respectively. Hiney et al. ( 1994 ) reiterated the view that Aer.
salmonicida may colonise mucus, gills and also fi ns. Also, these workers considered that the intestine may well be the primary location of Aer. salmonicida in
Atlantic salmon with asymptomatic infections. In addition, there are other considerations that may contribute to the problems experienced. For instance, parallels
might be drawn from studies on the isolation methods used for other microorganisms, i.e. notably coliforms and enteric pathogens such as Salmonella spp.,
where it has been observed that a pre-enrichment step must be employed prior to use
of selective media that impose too stringent conditions on these organisms which
have been stressed, injured or are too sensitive to selective agents, and, thus, are
rendered unrecoverable by selective methods alone (Geldreich 1977 ; Kaper et al.
1977 ; Olson 1978 ). Alternatively, it can be postulated that Aer. salmonicida follows
the pattern of certain other types of micro-organisms which are extremely diffi cult
to detect in the natural environment by means of routine bacteriological procedures
(i.e. plate counts) and, hence, have been assumed to be absent from these environments. Stevenson ( 1978 ) commented on adjustments made by bacteria, which
enable the organisms to survive in the variable and often stressful conditions
imposed upon them by existence in natural aquatic systems. He suggested that bacteria surmount changes in their environment, including varying degrees of solar
input, temperature, availability of nitrogen and dissolved oxygen, by entering a state
of dormancy defi ned as ‘any rest period or reversible interruption of the phenotypic
development of an organism’ (Sussman and Halvorson 1966 ). Thus, the possibility
that Aer. salmonicida, in the natural environment outside a fi sh host, assumes a
physiological state such that it cannot be recovered on agar plates used for primary
isolation should not be discounted. There is tentative evidence that the situation for
Aer. salmonicida is similar to that of a related pathogen, V. cholerae. In the survival
of V. cholerae in aquatic microcosms, Singleton et al. ( 1982a , b ) reported that certain combinations of environmental parameters, i.e. sub-optimal salinities and low
nutrient concentrations, not only affected multiplication of V. cholerae populations
but also recoverability of the cells. Thus, these investigators found that V. cholerae
cells were observed using acridine orange staining in conjunction with epifl uorescence microscopy when culturable cells were not detected. Further work on the
theme of a ‘non-recoverable’ stage of existence for bacterial populations which,
however, remain viable was done by Xu et al. ( 1982 ), also for V. cholerae. They
used direct viable counting, a procedure allowing estimation of substrateresponsiveness, i.e. viable cells, using microscopy. This method revealed that a signifi cant proportion of the non-culturable cells were, in fact, viable.
5 Aeromonadaceae Representative (Aeromonas salmonicida)
Number of Samples Revealing Aer. salmonicida
15/100 gill samples revealed the pathogen at 6.3 × 10
2 –1 × 10
4 /g
19/100 mucus samples revealed the pathogen at 9.1 × 10
2 –1.7 × 10
4 /g
Using CBB, Cipriano et al. ( 1996b ) reported Aer. salmonicida in higher numbers
from the mucus than kidney of six salmon. Thus, the populations of Aer. salmonicida were in the range of 1.1 × 10
3 to 1.8 × 10
7 /g and 1.0 × 10
3 to 1.4 × 10
7 /g for
mucus and kidney, respectively. Hiney et al. ( 1994 ) reiterated the view that Aer.
salmonicida may colonise mucus, gills and also fi ns. Also, these workers considered that the intestine may well be the primary location of Aer. salmonicida in
Atlantic salmon with asymptomatic infections. In addition, there are other considerations that may contribute to the problems experienced. For instance, parallels
might be drawn from studies on the isolation methods used for other microorganisms, i.e. notably coliforms and enteric pathogens such as Salmonella spp.,
where it has been observed that a pre-enrichment step must be employed prior to use
of selective media that impose too stringent conditions on these organisms which
have been stressed, injured or are too sensitive to selective agents, and, thus, are
rendered unrecoverable by selective methods alone (Geldreich 1977 ; Kaper et al.
1977 ; Olson 1978 ). Alternatively, it can be postulated that Aer. salmonicida follows
the pattern of certain other types of micro-organisms which are extremely diffi cult
to detect in the natural environment by means of routine bacteriological procedures
(i.e. plate counts) and, hence, have been assumed to be absent from these environments. Stevenson ( 1978 ) commented on adjustments made by bacteria, which
enable the organisms to survive in the variable and often stressful conditions
imposed upon them by existence in natural aquatic systems. He suggested that bacteria surmount changes in their environment, including varying degrees of solar
input, temperature, availability of nitrogen and dissolved oxygen, by entering a state
of dormancy defi ned as ‘any rest period or reversible interruption of the phenotypic
development of an organism’ (Sussman and Halvorson 1966 ). Thus, the possibility
that Aer. salmonicida, in the natural environment outside a fi sh host, assumes a
physiological state such that it cannot be recovered on agar plates used for primary
isolation should not be discounted. There is tentative evidence that the situation for
Aer. salmonicida is similar to that of a related pathogen, V. cholerae. In the survival
of V. cholerae in aquatic microcosms, Singleton et al. ( 1982a , b ) reported that certain combinations of environmental parameters, i.e. sub-optimal salinities and low
nutrient concentrations, not only affected multiplication of V. cholerae populations
but also recoverability of the cells. Thus, these investigators found that V. cholerae
cells were observed using acridine orange staining in conjunction with epifl uorescence microscopy when culturable cells were not detected. Further work on the
theme of a ‘non-recoverable’ stage of existence for bacterial populations which,
however, remain viable was done by Xu et al. ( 1982 ), also for V. cholerae. They
used direct viable counting, a procedure allowing estimation of substrateresponsiveness, i.e. viable cells, using microscopy. This method revealed that a signifi cant proportion of the non-culturable cells were, in fact, viable.
5 Aeromonadaceae Representative (Aeromonas salmonicida)