
 aeromonas hydrophila and plesiomonas shigelloides are gram - negative bacilli within the families aeromonadaceae and enterobacteriaceae , respectively .
although the two bacteria occupy separate taxonomic niches , they share attributes in being widely distributed in freshwater , estuarine , and marine environments , and both have been recognized to cause systemic infection in immunocompromised hosts and to be possible causes of diarrheal disease [ 1 , 2 ] .
water and food serve as vehicles of transmission for both pathogens . at present , aeromonads are not universally accepted as true enteropathogens . while some reports have ascribed   or refuted   an etiologic role to aeromonads in diarrheal disease , others [ 5 , 6 ] have postulated that only certain subsets of aeromonads equipped with genes for enterotoxicity are significantly associated with diarrhea .
several factors have contributed to uncertainty regarding the enteropathogenicity of aeromonads : the lack of recognized outbreaks with clonally distinct isolates recovered from patients and an implicated source ; infrequent documentation of person - to - person transmission ; a paucity of evidence demonstrating experimental pathogenicity in humans ; absence of a good animal model to carry out infection studies ; in some reports , overlapping prevalence of aeromonads in patients with diarrhea compared to asymptomatic individuals . in contrast , p. shigelloides has been identified as the cause of outbreaks of diarrheal disease [ 9 , 10 ] and is known to play an etiologic role in travelers ' diarrhea , dysentery , and diarrhea in pediatric   and adult   outpatients .
relatively few studies have compared the clinical features of diarrheal illness produced by a. hydrophila and p. shigelloides with those of more established enteric pathogens [ 15 , 16 ] .
such a comparison conducted in a region with a large number of illnesses could offer insight into clinical nuances associated with each agent .
accordingly , we compared the clinical features of diarrheal disease in persons infected solely with a. hydrophila or p. shigelloides with those in persons infected solely with vibrio cholerae non - o1 or vibrio parahaemolyticus .
we chose the latter pathogens for comparison because , like a. hydrophila and p. shigelloides , both are transmitted by food and water , infect a wide age spectrum of persons , and were shown to be prevalent in our study population , patients treated at the international centre for diarrhoeal disease research in dhaka , bangladesh ( icddr , b ) .
for over four decades , the international centre for diarrhoeal disease research in bangladesh has addressed leading health - related problems through patient care , collaborative research , and extension activities . as a component of its surveillance activities , in 1979 ,
the centre initiated a systematic sampling program wherein every 25th patient presenting to icddr , b with diarrheal disease was studied in detail for the microbiologic etiology of illness . employing standard microbiologic procedures , from 1996 through 2001 ,
diarrheal stools were tested routinely for a. hydrophila and p. shigelloides in addition to salmonella , shigella , vibrio cholera o1 , v. parahaemolyticus , v. cholerae non - o1 , other vibrios , e. coli ( enterotoxigenic , enteropathogenic , enteroaggregative , enteroinvasive , and shiga - toxin - producing ) , campylobacter , rotavirus , entamoeba , giardia , and cryptosporidium .
stool samples specifically were enriched in bile broth and subcultured onto taurocholate - tellurite - gelatin agar ( ttga ) .
transparent oxidase - positive presumptive colonies were tested with vibrio polyvalent and vibrio 0139 antisera , and those that tested negative were identified further based on sugar fermentation , amino acid content , and ability to grow in broth of varying salt concentrations .
isolates were identified as a. hydrophila based on sugar fermentation results ( esculin , salicin , arabinose , mannitol , sucrose , and inositol ) , amino acid content ( arginine , lysine , and ornithine ) , and ability to grow in broth of varying sodium chloride concentrations ( 0% , 6.5% , or 8% ) .
further characterization of isolates was undertaken using vibriostatic compound disc 0129 ( 150  g ) and the analytical profile index ( api ) , as necessary .
we analyzed microbiologic results for the period 1996 through 2001 along with select responses to a standard questionnaire administered to patients with diarrheal disease diagnosed and treated at icddr , b during the same period . among other items ,
the questionnaire solicited information regarding demographics , clinical features of illness , and microscopic evaluation of diarrheal specimens .
pairwise assessments of responses for patients infected with a. hydrophila or p. shigelloides were compared with those for patients infected with v. cholera non - o1 or v. parahaemolyticus using a chi - square or fisher 's exact test to assess differences of statistical significance ( p value < 0.05 ) .
we hypothesized that if a. hydrophila and p. shigelloides were enteropathogens , responses for patients with diarrheal disease from whom these two organisms were recovered from stool specimens would be similar to responses from persons infected with v. cholerae non - o1 or v. parahaemolyticus . for a variable called  outcome  in the questionnaire
, patients were asked to specify subjectively whether they felt  cured  prior to discharge or whether the illness was  continuing .    using daily temperature and rainfall data for dhaka for the entire study period obtained from the government of people 's republic of bangladesh , bangladesh meteorological department ( climate division ) , we assessed correlations between total monthly rainfall and mean monthly temperature with the number of patients treated at icddr
, b from whom a stool culture yielded only one of the following enteric pathogens : aeromonas hydrophila , plesiomonas shigelloides , vibrio cholerae non - o1 , or vibrio parahaemolyticus ; correlations were calculated using a threshold for statistical significance defined by a p value of less than 0.05 .
  from 1996 through 2001 , stool specimens were collected from 13,970 patients with diarrheal disease for assessment of the pathogens listed above . among these patients ,
a. hydrophila was the sole organism recovered from 189 ( 1.4% ) patients , p. shigelloides from 253 ( 1.8% ) patients , v. cholerae non - o1 from 99 patients ( 0.7% ) , and v. parahaemolyticus from 126 ( 0.9% ) patients .
the majority of patients were males ( table 1 ) . patients with a. hydrophila infection
were younger than those infected by p. shigelloides , v. cholerae non - o1 , or v. parahaemolyticus infections .  
patients with all four types of infections exhibited similar frequencies of fever ( temperature > 37.8c ) , stools characterized as watery , stools containing visible mucus , maximum number of stools per 24 hours , and duration of illness ( table 1 ) . on the other hand ,
dehydration was observed more often among patients with v. cholerae non - o1 or v. parahaemolyticus infection , both groups of which received intravenous rehydration more often than did patients from whom a. hydrophila or p. shigelloides was recovered . in general , patients from whom v. parahaemolyticus was recovered experienced the most severe infections as evidenced by the higher frequency of vomiting and production of stools with visible blood , and the higher percentage of stool specimens for which red or white blood cell counts exceeded 20 cells per high power field ; in addition , patients infected with v. parahaemolyticus were less likely than their counterparts to report a subjective sense of being cured at the time of discharge .  
although pathogens were isolated throughout the year , the monthly number of infections peaked in august for a. hydrophila , p. shigelloides , and v. cholerae non - o1 ( figure 1 ) .
the number of infections with v. cholerae non - o1 was correlated directly with total monthly rainfall and mean monthly temperature , while the number of v. parahaemolyticus infections was correlated with mean monthly temperature ( table 2 ) ; no significant correlations were observed with either meteorological variable and a. hydrophila or p. shigelloides .
in bangladesh , patients with diarrhea severe enough to require hospitalization were occasionally infected with a. hydrophila or p. shigelloides .
clinically , these patients shared a number of features with those infected with v. cholerae non - o1 or v. parahaemolyticus .
for example , fever ( temperature > 37.8c ) was rare , mucus was frequently visible in stools , a similar maximum number of stools per 24 hours was reported , and the duration of illness was similar .
moreover , patients infected with a. hydrophila , p. shigelloides , and v. cholerae non - o1 were similar in terms of the frequency with which > 20 red blood cells or white blood cells were present per high field on stool examination , as well as in terms of the subjective proportions who deemed themselves to be cured at the time of discharge .  
on the other hand , there appeared to be a gradation of severity such that patients infected with a. hydrophila or p. shigelloides experienced the least severe illness , while the severity of infection was somewhat greater for v. cholera non - o1-infected individuals , and most severe for patients with v. parahaemolyticus infection . while a greater proportion of patients with the four infections we studied were males , a finding similar to a report describing shigella infections in bangladesh , those infected with a. hydrophila were significantly younger than their counterparts .   otherwise , with regard to the visibility of mucus or blood in stools and the presence of > 20 red blood cells or white blood cells per high power field , patients infected with a. hydrophila and p. shigelloides resembled those infected with v. cholera non - o1 .
moreover , the proportion of patients whose infections were deemed to have been cured was similar for these three groups . however , intravenous hydration as an added component to oral rehydration therapy was used less often among patients with a. hydrophila or p. shigelloides infections compared to those infected with the vibrios .   in light of our findings that patients infected with a. hydrophila or p. shigelloides displayed clinical features that were , in large part , similar to those infected with v. cholera non - o1 , an organism recognized as a true enteropathogen
, we believe there is reason to ascribe a causative role for a. hydrophila in diarrheal disease . as others have stated
[ 1 , 6 ] , certain strains of a. hydrophila likely possess genetic attributes that facilitate the production of a secretory diarrhea in susceptible individuals .
supporting this concept was a study of the distribution of a. hydrophila enterotoxins alt , ast , and act in children with and without diarrhea in bangladesh ; the number of isolates positive for both the alt and ast genes was significantly higher for diarrheal children than for control children , suggesting the enterotoxins produced by the two genes may act synergistically to induce severe diarrhea .
our results corroborate a role for p. shigelloides as an etiologic agent of diarrhea as well , an organism that has been associated with outbreaks of diarrhea [ 9 , 10 ] and , in addition , has been shown to elaborate an enterotoxin that causes elongation of chinese hamster ovary cells by activating adenylate cyclase with a resulting increase in cyclic amp , a sequence of physiologic events characteristic of enterotoxins produced by vibrio cholerae and enterotoxigenic e. coli .  
we observed several differences in the epidemiology of the four pathogens assessed here . whereas a. hydrophila , p. shigelloides , and v. cholerae non - o1 infections were diagnosed commonly in infants , v. parahaemolyticus infections were distinctly uncommon in this age group , being most often diagnosed in persons aged 1544 years , a pattern described for v. parahaemolyticus cases in canada as well .
the pathogens exhibited divergences in terms of seasonal occurrence too , with no correlation seen between rainfall and temperature with either a. hydrophila or p. shigelloides infections .
in contrast , both v. cholerae non - o1 and v. parahaemolyticus were correlated with temperature , and v. cholerae non - o1 was correlated with rainfall as well . the lack of an observed correlation between a. hydrophila and p. shigelloides with either rainfall or temperature differs from the results of several other studies .
for example , increased recoveries of aeromonas from stool specimens have often been reported during warmer months , a time when concentrations of mesophilic organisms may increase in aquatic environments . in an investigation of a waterborne outbreak of p. shigelloides infections in japan , investigators reported that the recovery of p. shigelloides from environmental samples , and mud in particular , was most common during the warmer months .
it is possible that absence of a correlation between a. hydrophila and p. shigelloides infections with either rainfall or temperature in bangladesh is a result of conditions characteristic of bangladesh , including climatic , geographic , socioeconomic , sanitary , and host factors .  
a limitation of the present study was that not all known microbial causes of diarrhea were assessed in patients presenting to icddr , b from 1996 to 2001 .
thus , it is possible that pathogens other than those screened were , in fact , responsible for a proportion of cases attributed to a. hydrophila and p. shigelloides .
however , if this was the case , the same limitation might apply to patients from whom v. cholerae non - o1 or v. parahaemolyticus were recovered in pure culture .
arguing against this possibility are clinical and epidemiologic features described here that parallel findings from previous investigations of these two vibrios [ 15 , 24 , 25 ] .
thus , if a substantial number of the illnesses described here were truly sole infections , our paper adds to the evidence of p. shigelloides being a cause of diarrhea and strengthens the hypothesis that certain strains of a. hydrophila may be true enteropathogens as well .
 using data from the international centre for diarrhoeal disease research , bangladesh ( icddr , b ) from 1996 to 2001 , we compared the clinical features of diarrhea in patients with stool specimens yielding only a. hydrophila ( 189 patients ; 1.4% of 13,970 patients screened ) or p. shigelloides ( 253 patients ) compared to patients with sole v. cholerae non - o1 infection ( 99 patients ) or v. parahaemolyticus infection ( 126 patients ) . patients exhibited similar frequencies of fever ( temperature > 37.8c ) , stools characterized as watery , and stools containing visible mucus . 
 dehydration was observed more often among patients with v. parahaemolyticus or v. cholerae non - o1 infection . 
 compared to patients infected with v. parahaemolyticus , those with a. hydrophila , p. shigelloides , or v. cholerae non - o1 infection were less likely to report visible blood in the stool and , on microscopic examination , less likely to exhibit stool red blood cell and white blood cell counts exceeding 20 cells per high power field . 
 the proportion of patients reporting subjective cure at the time of discharge was significantly smaller for those infected with v. parahaemolyticus . 
 these findings suggest that a. hydrophila and p. shigelloides produce diarrheal disease that is less severe than that resulting from infection with v. cholerae non - o1 or v. parahaemolyticus . 
1. Introduction
2. Methods
3. Results
4. Discussion