J. Michael Janda

Evolving Concepts Regarding the Genus Aeromonas: An Expanding Panorama of Species, Disease Presentations, and Unanswered Questions

It has been almost 10 years since a major review on the association of Aeromonas with human disease has been published. During that period the number of valid species in the genus has grown to 14, with a new family (Aeromonadaceae) established to house this genus. Despite this explosion in the number of new genomospecies, only five (Aeromonas hydrophila, A. caviae, A. veronii, A. jandaei, and A. schubertii) are currently recognized as human pathogens. New syndromes attributed to this genus include hemolytic uremic syndrome, burn-associated sepsis, and a variety of respiratory tract infections, including epiglottitis. Convincing evidence suggests that some aeromonads do cause gastroenteritis, but it is presently unclear whether many of the strains isolated from feces are involved in diarrheal disease. Many questions regarding this genus remain unanswered.

Aeromonas and Plesiomonas.

Publisher Summary Aeromonas species are now recognized as important causes of intestinal and extraintestinal illnesses in humans, which include bacteraemia and wound infections. Aeromonas are also responsible for major die-off of pond fish, carp, and salmonids and occasionally they can cause sporadic and epizootic disease in mammals, reptiles, amphibian, and birds. Identification charts for Aeromonas species frequently or occasionally found in clinical material are outmined in this chapter. Aeromonas septicaemia is almost invariably a disease associated with debilitated hosts, especially in those with haematological malignancies or hepatobiliary disease. The first well-documented cases of P. shigelloides infection involved a case of cellulitis associated with a head wound. Most often, this unusual pathogen appears to cause sporadic cases of gastroenteritis and on rare occasions may be associated with outbreak disease. Less frequently plesiomonads are implicated in systemic infections, such as septicaemia in persons with one or more underlying complications.. The main routes of transmission include freshwater and marine sources and inhabitants of those niches including fish, shellfish, amphibia, and reptiles. Control of such low-incidence disease is dependent upon better hygienic conditions including the sanitary treatment of potable water sources and the appropriate cooking of shellfish.

Evolutionary Genetics of a New Pathogenic Escherichia Species: Escherichia albertii and Related Shigella boydii Strains

A bacterium originally described as Hafnia alvei induces diarrhea in rabbits and causes epithelial damage similar to the attachment and effacement associated with enteropathogenic Escherichia coli. Subsequent studies identified similar H. alvei-like strains that are positive for an intimin gene (eae) probe and, based on DNA relatedness, are classified as a distinct Escherichia species, Escherichia albertii. We determined sequences for multiple housekeeping genes in five E. albertii strains and compared these sequences to those of strains representing the major groups of pathogenic E. coli and Shigella. A comparison of 2,484 codon positions in 14 genes revealed that E. albertii strains differ, on average, at approximately 7.4% of the nucleotide sites from pathogenic E. coli strains and at 15.7% from Salmonella enterica serotype Typhimurium. Interestingly, E. albertii strains were found to be closely related to strains of Shigella boydii serotype 13 (Shigella B13), a distant relative of E. coli representing a divergent lineage in the genus Escherichia. Analysis of homologues of intimin (eae) revealed that the central conserved domains are similar in E. albertii and Shigella B13 and distinct from those of eae variants found in pathogenic E. coli. Sequence analysis of the cytolethal distending toxin gene cluster (cdt) also disclosed three allelic groups corresponding to E. albertii, Shigella B13, and a nontypeable isolate serologically related to S. boydii serotype 7. Based on the synonymous substitution rate, the E. albertii-Shigella B13 lineage is estimated to have split from an E. coli-like ancestor approximately 28 million years ago and formed a distinct evolutionary branch of enteric pathogens that has radiated into groups with distinct virulence properties.

The Genus Hafnia: from Soup to Nuts

SUMMARY The genus Hafnia, a member of the family Enterobacteriaceae, consists of gram-negative bacteria that are occasionally implicated in both intestinal and extraintestinal infections in humans. Despite the fact that the genus currently contains only a single species (H. alvei), more extensive phylogenetic depth (two or more species) is apparent based upon DNA relatedness and 16S rRNA gene sequencing studies. Hafnia causes a variety of systemic infections, including septicemia and pneumonia; however, its role as a gastrointestinal pathogen is controversial. Many of the data supporting a role for hafniae as enteric pathogens were incorrectly attributed to this genus rather than to the actual pathogen, Escherichia albertii. There are numerous gaps in our understanding of this genus, including ecologic habitats and population genetics, disease-producing role in animals, phenetic and genetic methods useful in distinguishing genomospecies within the H. alvei complex, and bona fide pathogenicity factors.

Rat-bite fever (Streptobacillus moniliformis): A potential emerging disease

OBJECTIVESnTo determine the relative prevalence of human infections attributable to Streptobacillus moniliformis in California over the past 3 decades.nnnMETHODSnA retrospective analysis of all the data collected was conducted on S. moniliformis cultures identified by the Microbial Diseases Laboratory (MDL) from January 1970 to December 1998.nnnRESULTSnInformation on a total of 45 S. moniliformis isolates was analyzed. Overall, 91% of the isolates were from human sources; 58% were received since 1990. These strains were divided almost equally between males and females, with 50% of the isolates from patients 9 years old or younger. In 75% of the cases of human infections where a diagnosis was given, rat-bite fever (RBF) was suspected; 83% of these suspected cases involved either a known rat bite or exposure to rodents.nnnCONCLUSIONSnAs crowding becomes an increasing environmental reality, humans are more frequently being exposed to zoonotic diseases as a result of encounters with wild animals. Domesticated animals also are exposed more frequently to wild animals; thus, increasing human exposure to once rare zoonotic illnesses. Rat-bite fever is a disease that seems to be easily recognizable by clinicians, easily identified in the clinical laboratory (if suspected), and successfully treated when the appropriate therapy is administered. Physicians should consider RBF as a possible diagnosis when fever, rash, and exposure to rats are part of the patients history.

The Thermostable Direct Hemolysin Gene (tdh) of Vibrio hollisae Is Dissimilar in Prevalence to and Phylogenetically Distant from the tdh Genes of Other Vibrios: Implications in the Horizontal Transfer of the tdh Gene

Vibrio hollisae strains isolated recently from patients in various locations were examined for the presence of the thermostable direct hemolysin gene (tdh) using nucleic acid hybridization and polymerase chain reaction assays. The results were consistent with the previous finding that all strains of V. hollisae carry the tdh gene. In contrast, the tdh gene has been detected in a minority of strains for other Vibrio species (V. parahaemolyticus, V. cholerae non‐O1, and V. mimicus). Detailed phylogenetic analysis showed that the tdh genes of the non‐V. hollisae species were very closely related to each other and that the tdh gene of V. hollisae was distantly related to the tdh genes of the non‐V. hollisae species. These results and the proposed insertion sequence‐mediated tdh transfer mechanism suggest that the tdh gene may have been maintained stably in V. hollisae and that the tdh genes of the non‐V. hollisae species may have been involved in recent horizontal transfer.

Phenotypic and genotypic properties of the genus Hafnia

The present study characterised 73 Hafnia alvei isolates and five Escherichia isolates (originally identified as H. alvei) isolated from cases of diarrhoeal disease by the International Centre for Diarrhoeal Disease Research Branch (ICDDRB) in Bangladesh. Based upon the hydrolysis of arbutin and aesculin and the fermentation of salicin and D-arabinose, four distinct biotypes could be recognised among the 73 H. alvei isolates tested; biotype 1 (D-(-)-arabinose-positive only) accounted for 75% of all isolates analysed. Hydrolysis of aglycone compounds such as arbutin, salicin and aesculin appeared to be associated with expression of beta-glucosidase activity. ICDDRB isolates, when compared with type or reference strains of H. alvei, were shown not to belong to the genus Hafnia based upon resistance to Hafnia-specific bacteriophage 1672, possession of the phoE gene, expression of glutamate decarboxylase activity and significant 16S rDNA sequence divergence (approximately 8%) from the type strain, ATCC 13337T. True H. alvei strains, implicated in outbreaks of diarrhoeal disease in Canada, lacked the eaeA gene in contrast to ICDDRB isolates. Twenty-two H. alvei isolates were selected for further study. Based upon partial 16S rDNA sequencing, these 22 isolates fell into two genomic groups (genomospecies), identical to DNA groups previously established by DNA hybridisation studies. Markers such as motility, biotype, or enzymic or carbohydrate fermentation patterns did not correlate totally with DNA grouping, although malonate utilisation appeared to be the single best discriminatory phenotype. The results indicate that the genus Hafnia is heterogeneous and there do not appear to be any laboratory data available specifically linking these organisms to gastro-enteritis.

The Enterobacteria, Second Edition

The Enterobacteria, 2nd Edition provides a comprehensive examination of current research on every genus in the family Enterobacteriaceae. A host of diagnostic tables, reference charts, color plates, and various other laboratory resources, this revised reference is an important source of information from microbiologists, lab directors, pathologists, and infectious disease specialists.nThe Enterobacteria, 2nd Edition presents complete medical information on each genus, including taxonomy, epidemiology, infectious disease spectrum, identification, pathogenicity, and antimicrobial susceptibility. Additionally, this new volume presents unique information on procedures and protocols for identification of atypical or unusual variants of each species. Select new features of the second edition are use of the genome sequence as a means of identification and classification, coverage of the reclassified genus Plesiomonas, and a brand-new appendix containing differential biochemical tables.nElectronic only, 411 pages, full-color insert, illustrations, index.

Plesiomonas and human disease

Abstract A number of antimicrobial agents are available to treat either intestinal or extraintestinal infections. If deemed necessary, appropriate antimicrobial agents shorten the clinical course of diarrhea (5, 6). Suitable oral agents for treating Plesiomonas intestinal disease could include the quinolones or trimethoprim-sulfamethoxazole. Aminoglycosides, except for netilmicin, are not considered first-line drugs for therapy. The broad-spectrum penicillins alone should never be used to treat an extraintestinal infection unless appropriate testing indicates susceptibility to these agents. Although considerable progress in understanding the role of P. shigelloides as a gastrointestinal pathogen has been made over the past several years, a number of hurdles remain. Foremost, an animal model to reproduce this disease needs to be developed. Secondly, reports on potentially new toxins and virulence factors need to be confirmed, and then their relationship to intestinal and extraintestinal disease needs to be defined. It is also unclear whether all P. shigelloides are equally virulent or whether strain-to-strain variation occurs. When there obstacles are circumvented, we will have a more accurate picture of the role of these gram-negative bacilli in human disease.

Immunochemical analysis and possible biological role of an Aeromonas hydrophila surface array protein in septicaemia

The biochemical, immunological, and biological properties of an S layer purified from an Aeromonas hydrophila strain (AH-342) involved in a case of bacteraemia were investigated. The S layer selectively removed from the cell surface was composed of a single acidic (pI 4.56) protein subunit (surface array protein, SAP) with a molecular mass of approximately 52 kDa. Amino acid analysis of this 52 kDa protein indicated a molecule composed of 498 amino acids with 46% hydrophobic residues. No cysteine residues were detected. The first 35 residues of the N-terminus were sequenced by Edman degradation; only 4-24% homology was noted between this sequence and those previously published for SAPs of Aeromonas salmonicida (A450) and a strain of A. hydrophila (TF7) originally isolated from a moribund fish. Polyclonal antibodies raised against AH-342 SAP were genospecific, reacting only against S layers produced by A. hydrophila strains and not those from Aeromonas veronii. Acute serum from the bacteraemic patient from whom AH-342 was isolated reacted strongly with the SAP of AH-342 in immunoblot studies. Purified SAP, when intraperitoneally co-inoculated with SAP- strains of A. hydrophila into Swiss-Webster mice, could reduce the 50% lethal dose by approximately 30-70 fold. The results suggest that the SAP of A. hydrophila strains may play an important role in systemic dissemination after invasion through the gastrointestinal mucosa.