Lack of association between presentation of diarrhoeal symptoms and faecal isolation of Aeromonas spp. amongst outpatients in Hong Kong

Reassessment of the Enteropathogenicity of Mesophilic Aeromonas Species

Cases of Aeromonas diarrhea have been described all over the world. The genus Aeromonas includes ca. 30 species, of which 10 have been isolated in association with gastroenteritis. The dominating species that account for ca. 96% of the identified strains are Aeromonas caviae, A. veronii, A. dhakensis, and A. hydrophila. However, the role of Aeromonas as a true enteropathogen has been questioned on the basis of the lack of outbreaks, the non-fulfillment of Koch's postulates and the low numbers of acute illnesses in the only existing human challenge study. In the present study we reassess the enteropathogenicity of Aeromonas using dose response models for microbial infection and acute illness. The analysis uses the data from the human challenge study and additional data from selected outbreak investigations where the numbers exposed and the dose were reported, allowing their inclusion as "natural experiments". In the challenge study several cases of asymptomatic shedding were found (26.3%, 15/57), however, only 3.5% (2/57) of those challenged with Aeromonas developed acute enteric symptoms (i.e., diarrhea). The "natural experiments" showed a much higher risk of illness associated with exposure to Aeromonas, even at moderate to low doses. The median dose required for 1% illness risk, was ~1.4 × 104 times higher in the challenge study (1.24 × 104 cfu) compared to natural exposure events (0.9 cfu). The dose response assessment presented in this study shows that the combined challenge and outbreak data are consistent with high infectivity of Aeromonas, and a wide range of susceptibility to acute enteric illness. To illustrate the outcomes, we simulate the risk associated with concentrations of Aeromonas found in different water and food matrices, indicating the disease burden potentially associated with these bacteria. In conclusion this study showed that Aeromonas is highly infectious, and that human susceptibility to illness may be high, similar to undisputed enteropathogens like Campylobacter or Salmonella.

Distribution, virulence-associated genes and antimicrobial resistance of Aeromonas isolates from diarrheal patients and water, China

OBJECTIVES

To determine the prevalence of Aeromonas infections in diarrheal patients, the distribution of virulence-associated genes and antibiotic resistance among different Aeromonas species in China.

METHODS

We conducted continual active surveillance aimed on Aeromonas from diarrheal patients and aquatic samples. Aeromonas strains were identified by biochemical tests, further confirmed to species level by a multilocus phylogenetic analysis. Potential virulence genes were detected by PCR. Antibiotics susceptibility testing was carried based on the minimal inhibitory concentration.

RESULTS

From 5069 samples (stool specimens, n = 4529; water samples, n = 540) in China, 257 Aeromonas isolates [stools, n = 193 (4.3%); water, n = 64 (11.9%)] were identified by biochemical tests. The most common species from stools and water were Aeromonas veronii (42.5%) and Aeromonas caviae (37.5%), respectively. Distribution of five potential genes were significantly different between stool and water samples, two genes (ast and alt) were higher in stool than in water samples (P < 0.01). Meanwhile, three species (A. veronii, A. caviae and Aeromonas aquariorum) account for the six most prevalent combination patterns of potential genes. Furthermore, strains resistant to nine antibiotics was markedly higher in strains isolated from water than those from stools (P ≤ 0.003); in contrast, resistance to only two antibiotics was higher in strains isolated from stools compared to those from water. In addition, strains containing multiple antibiotic resistance (MAR) from stools (8.6%; 16/187) and water (30.2%; 19/63) were resistant to ten or more antibiotics.

CONCLUSION

Our study highlights the multiple factors involved in the pathogenesis of Aeromonas and reveals that environmental Aeromonas has acquired a wide range of MAR compared to those from clinical sources.

Clinical and Epidemiologic Features of Diarrheal Disease due to Aeromonas hydrophila and Plesiomonas shigelloides Infections Compared with Those due to Vibrio cholerae Non-O1 and Vibrio parahaemolyticus in Bangladesh

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 >37.8°C), 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.

The genus Aeromonas: Taxonomy, pathogenicity, and infection. Clin Microbiol Rev. 2010;23:35-73. [PMID: 20065325 DOI: 10.1128/cmr.00039-09]

Over the past decade, the genus Aeromonas has undergone a number of significant changes of practical importance to clinical microbiologists and scientists alike. In parallel with the molecular revolution in microbiology, several new species have been identified on a phylogenetic basis, and the genome of the type species, A. hydrophila ATCC 7966, has been sequenced. In addition to established disease associations, Aeromonas has been shown to be a significant cause of infections associated with natural disasters (hurricanes, tsunamis, and earthquakes) and has been linked to emerging or new illnesses, including near-drowning events, prostatitis, and hemolytic-uremic syndrome. Despite these achievements, issues still remain regarding the role that Aeromonas plays in bacterial gastroenteritis, the extent to which species identification should be attempted in the clinical laboratory, and laboratory reporting of test results from contaminated body sites containing aeromonads. This article provides an extensive review of these topics, in addition to others, such as taxonomic issues, microbial pathogenicity, and antimicrobial resistance markers.

Emerging infections of the gastrointestinal tract

Infections account for significant GI morbidity and mortality worldwide. New organisms are being identified, associated with diarrhoeal illness and some with other gastrointestinal illness as well. Among GI viruses, Sapovirus is now recognised to cause diarrhoea, especially in children. A hypervirulent strain of Clostridium difficile has caused epidemics in many countries. Newly identified bacterial species that may cause diarrhoea include Campylobacter concisus, Arcobacteria, Edwardsiella tarda, Aeromonas, Plesiomonas and Laribacter. Helicobacteria are reviewed, as well as the role of gastric acid suppression in predisposing to enteric infections.