Implementation of EPA Revised Cancer Assessment Guidelines: Incorporation of Mechanistic and Pharmacokinetic Data

A workshop entitled "Implementation of EPA Revised Cancer Assessment Guidelines: Incorporation of Mechanistic and Pharmacokinetic Data" was held in Anaheim, California, in 1996 at the 35th Annual Meeting of the Society of Toxicology (SOT). This workshop was jointly sponsored by the Carcinogenesis, Risk Assessment, and Veterinary Specialty Sections of the SOT. The thrust of the workshop was to discuss the scientific basis for the revisions to the EPA Guidelines for cancer assessment and EPA's plans for their implementation. This is the first revision to the original EPA guidelines which have been in use by EPA since 1986. The principal revisions are intended to provide a framework for an increased ability to incorporate biological data into the risk assessment process. Two cases were presented, for chloroform and triclioroethylene, that demonstrated the use of the revised guidelines for specific cancer risk assessments. Using these new guidelines, nonlinear margin of exposure analyses were proposed for these chemicals instead of the linearized multistage model previously used by the EPA as the default method. The workshop participants generally applauded the planned revisions to the EPA guidelines. For the most part, they considered that the revised guidelines represented a positive step which should allow for and encourage the use of biological information in the conduct of cancer risk assessments. Several participants cautioned however that the major problem with cancer risk assessments would continue to be the inadequacy of available data on which to conduct more scientific risk assessments.

Enteropathogenicity of Aeromonas jandaei and A. trota

An experimental study of five isolates of Aeromonas jandaei and 12 of A. trota was carried out to examine if they produced an enterotoxic substance, and if so, to characterise that factor and to see if it caused any mucosal damage. Only two of the A. trota strains caused fluid accumulation in the initial rabbit ileal loop (RIL) tests. The remaining strains did so only after one to five sequential passages through RILs and once they caused a secretory response they showed a gradual enhancement of fluid outpouring after each subsequent passage. Inocula of approximately 1 x 10(5) viable cells and 0.25 ml of culture filtrate caused fluid accumulations comparable to those of toxigenic V. cholerae 569B. The enterotoxic factors of both organisms were inactivated when held at 56 degrees C for 20 min or 65 degrees C for 10 min and showed biological activity over a wide range of pH. The only histopathological change observed in the ileal loop was depletion of mucus from the goblet cells. These data thus indicate that strains of A. jandaei and A. trota may produce a heat-labile and pH-stable diarrhoeagenic substance that causes little or no damage to the intestinal mucosa, like that of other known heat-labile enterotoxins.

Enterotoxicity, haemolytic activity and antibiotic susceptibility of Aeromonas eucrenophila strains isolated from water and infected fish

Strains of A. eucrenophila isolated from fresh water (2 strains) and infected fish (4 strains) were tested for haemolytic activity and enterotoxicity and any correlation between them. Also, the resistance patterns of A. eucrenophila were tested especially in relation to ampicillin. None of the A. eucrenophila strains caused fluid accumulation in the initial tests, however, they did so only after one to four sequential passages through the gut of a susceptible host. All the strains of A. eucrenophila showed beta-haemolytic activities. Production of beta-haemolysin could be correlated with enterotoxicity. Since all the strains of A. eucrenophila were resistant to ampicillin, media containing this antibiotic may be used for their isolation from diverse sources.

Production of haemolysin and enterotoxin by Aeromonas jandaei and Aeromonas trota strains after animal passage

Five Aeromonas jandaei and 12 Aeromonas trota isolates were tested for the production of haemolysin and enterotoxin, and the correlation between these two properties. The majority (10 isolates) of the strains produced beta-haemolysis. The titres of haemolytic activity for both species were 8-64 HU/mL. In the initial ileal loop test, only two (A. trota) of the 17 isolates produced enterotoxin. One each of these 2 A. trota strains was beta-haemolytic and non-haemolytic. The remaining isolates of A. trota and A. jandaei included alpha-, beta- and non-haemolytic strains, and failed to cause any fluid accumulation in the initial tests, but did so after one-to-five sequential passages through the rabbit ileal loops. Three alpha- and 4 non-haemolytic strains switched over to the production of beta-haemolysis when they showed the positive ileal loop reaction. However, on repeated subcultures or on storage in the laboratory, all of them reverted back to their original alpha- or non-haemolytic character and no longer produced enterotoxin.

Haemolysin produced by Vibrio cholerae non-O1 is not enterotoxic

Of 28 isolates of Vibrio cholerae non-O1 (10 from diarrhoeal patients and 18 from environmental sources) examined for haemolytic activity and its correlation, if any, with enterotoxic activity, 24 showed haemolysis. The four non-haemolytic isolates showed haemolysis after consecutive passages through rabbit ileal loops (RILs). The titres of haemolytic activity were 4-64 HU/ml irrespective of their source. Eight (28.5%) of the non-O1 isolates caused fluid accumulation; six (25%) were haemolytic and two (50%) non-haemolytic. The remaining isolates showed enterotoxic activity after one-to-three consecutive passages through RILs irrespective of their haemolytic character and source. Environmental isolates caused significantly more fluid accumulation than the diarrhoeal isolates. All these isolates reverted to their original non-toxigenic character on repeated subculture or on storage in the laboratory, but continued to show haemolytic activity. The results of the present study indicate that V. cholerae non-O1 strains are potentially enterotoxigenic independent of their haemolytic character and source, and enterotoxin, not haemolysin, is the factor most likely to be responsible for their enterotoxic activity.

Production of the new cholera toxin by environmental isolates of Vibrio cholerae non-O1

One of five strains of Vibrio cholerae non-O1 isolated from environmental sources caused fluid accumulation in an initial rabbit ileal loop (RIL) test. The four strains that caused little or no accumulation of fluid gave a positive response after one-to-three consecutive passages through RILs. The amount of fluid produced increased after each passage. Filtrates of cultures of all five environmental isolates caused fluid accumulation similar to that produced by live cells. The enterotoxin showed a precipitin band with new cholera antitoxin and was neutralised completely by new cholera antitoxin diluted 1 in 32, indicating its close immunobiological relationship to the new cholera toxin. The present study indicates that V. cholerae non-O1 strains produce an enterotoxin that is similar to the new cholera toxin.

Development of an improved synthetic medium for a better production of the new cholera toxin and its immunological relationship with the toxin produced by Vibrio cholerae O139 strains

An improved synthetic medium (M4) comprising syncase medium supplemented with sodium chloride (1%) and sucrose (0.5%) pH adjusted to 7.4 was developed for a better production of the new cholera toxin (NCT). The culture filtrates prepared in the M4 medium caused significantly (P > 0.05) more fluid accumulation than that in syncase medium. Crude toxin, prepared in the M4 medium with V. cholerae O1 strains (X-392 and 2740-80) caused a reaction similar to that of the same amount of NCT (32 micrograms) prepared in the syncase medium. The neutralization of the optimal loop reacting dose of the NCT prepared in the M4 medium by anti-NCT raised against syncase prepared toxin indicates the release of the same kind of toxin in both media. These observations indicate that the modified M4 medium may be used for NCT preparation and further characterization. All the strains of Vibro cholerae O139 used in this study produced a toxin antigenically similar to NCT.

Attachment of non-culturable toxigenic Vibrio cholerae O1 and non-O1 and Aeromonas spp. to the aquatic arthropod Gerris spinolae and plants in the River Ganga, Varanasi

Non-cultivable, pathogenic O1 and non-O1 Vibrio cholerae and Aeromonas spp. were resuscitated from aquatic arthropods and plant homogenate respectively, by rabbit ileal loop (RIL) assay. These organisms adhered to the aquatic arthropod Gerris spinolae and various species of phytoplankton in the River Ganga, but failed to grow after direct inoculation on artificial media except for only 10 homogenates of the arthropod. The number of non-O1 V. cholerae and Aeromonas recovered on direct inoculation of G. spinolae homogenates were in the order of 10(5)-10(6) whereas those of the Ganga water were 10(2)-10(3) ml-1. A total of 119 strains of O1 and non-O1 V. cholerae and Aeromonas spp. (69 isolates from G. spinolae and 50 from aquatic plants) were recovered from the loop contents. The results indicate that production of the enzyme chitinase by O1 and non-O1 V. cholerae and Aeromonas spp. might facilitate their adsorption and multiplication on different species of zoo- and phyto-plankton. Most of the isolates were enterotoxic, haemolytic and resistant to different antibiotics. This study suggests that species of zoo- and phyto-planktons, until now not reported to be associated with O1 and non-O1 V. cholerae, may act as reservoirs of these organisms as well as different species of Aeromonas in a fresh-water riverine ecosystem.

Relationship between enterotoxicity and multiple drug resistance in Aeromonas spp

One hundred forty-seven isolates of Aeromonas spp., including 54 from diarrhoeal patients and 93 from the environmental sources, were examined for drug sensitivity and enterotoxin production. One hundred and fifteen (78%) isolates that included all A. hydrophila, 60% of A. sobria and 89% of A. caviae showed resistance to one or more antibiotic(s). Most (65%) of the resistant isolates caused fluid accumulation in rabbit ileal loops, of these 57% were A. hydrophila, 69% of A. sobria and 65% A. caviae, whereas only a few drug-sensitive isolates did so. Multiple drug-resistant isolates caused comparatively more fluid accumulation than the drug-sensitive isolates. Furthermore, significant difference of ileal fluid accumulation was observed with the increase in the number of resistance markers. This study suggests that there is some association between multiple drug resistance and enterotoxicity of Aeromonas strains, regardless of their source of isolation and species.

Influence of animal passage on haemolysin and enterotoxin production in Vibrio cholerae O1 biotype El Tor strains

Of 43 strains of Vibrio cholerae O1 biotype El Tor isolated over a span of almost three decades (1964-1990) from stools of children and adults with diarrhoea (25 isolates) and from sewage (three) and water from the river Ganges (15) examined for production of haemolysin and its correlation with enterotoxin production, 17 isolates showed haemolysis. The majority of isolates (26), including 68% of diarrhoeal and 50% of environmental origin, were non-haemolytic. The titre of haemolysin produced was 4-16 HU/ml, irrespective of the source of isolation. Haemolytic strains caused significantly more fluid accumulation than the non-haemolytic strains in the rabbit ileal loop (RIL) test. Twenty nine (67.4) V. cholerae biotype El Tor isolates--all the haemolytic and most (61.5%) of the non-haemolytic isolates tested--caused fluid accumulation. The remaining non-haemolytic strains that caused little or no accumulation of fluid did so after one to four consecutive passage(s) through RIL without change in haemolytic character; these strains required more consecutive passage through rabbit gut to show haemolysis. All these strains reverted to their original non-haemolytic character on repeated subculture or on storage in the laboratory but continued to show enterotoxic activity. The present study indicated that El Tor haemolysin is not responsible for fluid accumulation in rabbit gut.

Adherence of haemagglutinating and non-haemagglutinating clinical and environmental isolates of Aeromonas

Twelve haemagglutinating and non-haemagglutinating isolates of Aeromonas spp., comprising 6 each of clinical and environmental origin, were examined for their ability to adhere to rabbit intestinal epithelium, for inhibition of adhesion with sugars, and for delineation of the portion of intestine, jejunum, or ileum that is most susceptible to adhesion. Although the environmental isolates of Aeromonas haemagglutinated human erythrocytes that were inhibited by D-mannose and/or L-fucose, the majority of the clinical isolates of Aeromonas adhered to rabbit intestinal epithelium in almost equal proportions regardless of their haemagglutination (HA) properties, species designation, and source of isolation. Adhesion of both haemagglutinating and non-haemagglutinating isolates of Aeromonas was inhibited by sugars; however, the ability of sugar inhibition to adhere was similar to that observed with HA. This study suggests that adhesion is probably mediated by a variety of pilus or non-pilus colonisation factors which may or may not be a haemagglutinin. The jejunum was found to be more susceptible to adhesion than the ileum. However, no appreciable difference was observed in the number of adhered bacteria to adjacent loops.

Haemagglutinating activity, serum sensitivity and enterotoxigenicity of Aeromonas spp

Of 97 isolates of Aeromonas spp. that were examined for haemagglutination (HA) and enterotoxigenicity, 35 were from clinical and 62 from environmental sources; 66 of them were also screened for sensitivity to normal human serum (NHS). HA was caused by 44 isolates (45%); it was unrelated to the source of the strain, but it was caused by a higher proportion of the isolates of A. hydrophila than of A. sobria or A. caviae. Of the haemagglutinating strains, 82% were enterotoxigenic, whereas most of the non-haemagglutinating strains were non-toxigenic when tested initially. All the latter became enterotoxin producers after serial passage through rabbit ileal loops, but without change in HA. Most (64%) of the isolates, including 68% of A. caviae (72% of clinical and 65% of environmental), were resistant to the bactericidal action of NHS. Most (92%) of the serum-sensitive strains were killed by activation of both the classical and alternate pathways of complement, the others only by the alternate pathway. Most (74%) of the serum-resistant strains caused fluid accumulation in the initial tests in ileal loops, regardless of species or source. Haemagglutinating and serum-resistant strains caused significantly more accumulation of fluid (p < 0.05) than non-haemagglutinating and serum-sensitive strains. This study shows partial correlation between HA or serum sensitivity and enterotoxigenicity, but the properties are probably not genetically linked.

Production of chitinase by enterotoxigenic Aeromonas species isolated from clinical and environmental sources

Thirty-six isolates of Aeromonas, 13 from children and 23 from the environment were tested for production of chitinase in culture supernatants. Thirty-four isolates of the three species (91% clinical and 96% environmental) produced constitutive chitinase. The environmental isolates elaborated significantly more (p < 0.005) of the enzyme than those from the children. Four of the environmental isolates also produced inducible chitinase. All isolates of Aeromonas were enterotoxic, however, the isolates producing inducible chitinase showed significantly higher enterotoxic activity. This study indicates that there is correlation between production of enterotoxin and chitinase in Aeromonas species.

Production of haemolysis and its correlation with enterotoxicity in Aeromonas spp

A total of 147 clinical and environmental isolates of Aeromonas that included 14 A. hydrophila, 60 A. sobria and 73 A. caviae strains was tested for haemolysin production and its correlation with enterotoxicity; 108 isolates produced beta-haemolysis. For A. hydrophila and A. sobria, titres of haemolysin were 16-128 HU/ml and for A. caviae, 16-64 HU/ml. In the ileal loop test, 82 (55.8%) strains of Aeromonas spp. produced enterotoxin. Of the beta-haemolytic strains, 72.7% of A. hydrophila, 58.6% of A. sobria and 68.6% of A. caviae isolates caused fluid accumulation in rabbit ileal loops. One strain each of alpha-haemolytic A. sobria and A. caviae, one of non-haemolytic A. sobria and nine of non-haemolytic A. caviae also caused a secretory response. The beta-haemolytic strains caused significantly more (p < 0.05) fluid accumulation than the alpha- and non-haemolytic isolates regardless of their species designation. The remaining 65 (44.2%) isolates belonging to the three species included alpha-, beta- and non-haemolytic strains: they failed to cause fluid accumulation in the initial experiments but did so after one to three consecutive passages through rabbit ileal loops. Two alpha- and 13 non-haemolytic strains switched to production of beta-haemolysis when they showed positive ileal loop reactions. However, on repeated subcultures or on storage in the laboratory, all of them reverted to their original haemolytic character and no longer produced enterotoxic activity.

Enterotoxicity of clinical and environmental isolates of Aeromonas spp

Of 147 isolates of three species of Aeromonas, 54 were from clinical and 93 from environmental sources. When tested for enterotoxin production, most of the isolates (56%) caused accumulation of fluid in rabbit ileal loops (RILs). Although large proportions of clinical and environmental isolates of A. caviae (55% and 65%, respectively) elicited such a response in RILs, isolates of A. hydrophila and A. sobria produced significantly more fluid (p less than 0.05). Furthermore, the environmental strains of A. hydrophila and A. sobria produced more fluid than the clinical isolates (p less than 0.05). The strains of Aeromonas spp. that caused little or no fluid accumulation in the initial experiments became enterotoxin producers after 1-3 passages through RILs, regardless of their source, and showed gradual enhancement of fluid outpouring after each passage. The present study suggests that all strains of these species of Aeromonas are potentially enterotoxigenic, whether from clinical or environmental sources.

Haemolysin and enterotoxin production by Aeromonas caviae isolated from diarrhoeal patients, fish and environment

Beta-haemolytic activity was shown by 46 (63%) of the 73 Aeromonas caviae strains isolated from diverse sources, such as diarrhoeal stools, fish ulcers and water in titres of 16-64 HU/ml. Only 2 strains showed alpha-haemolytic activity and the remaining 27% of them were nonhaemolytic. Live cells and culture filtrates of 60.3% of the A. caviae isolates caused accumulation of fluid in rabbit gut loops in the initial set of experiments. Of the 46 strains showing beta-haemolytic activity only 34 gave positive ileal loop reactions in the initial experiments. One of the 2 strains showed alpha-haemolytic activity and 9 of the 20 nonhaemolytic strains also caused fluid accumulation in the same set of experiments. Those strains that showed beta-haemolytic activity caused significantly more (p less than 0.01) fluid outpouring than the alpha- or nonhaemolytic isolates regardless of their sources of origin. Twenty-nine (39.7%) strains that showed alpha-, beta-, and nonhaemolytic activity and caused little or no fluid accumulation in initial experiments did so after 1-3 consecutive passages through rabbit gut. The nontoxic strain showing alpha- and non-haemolytic activity switched over to production of beta-haemolytic activity once their live cells gave positive loop reactions. However, on repeated subcultures or on preservation in the laboratory for 2-3 weeks, all of them reverted back to their original nontoxic haemolytic types, i.e. alpha- or nonhaemolytic activity.