1) Aeromonas is a genus of bacteria commonly found in freshwater and brackish aquatic environments that can cause disease in humans and other animals. It includes 17 known species, with A. hydrophila, A. caviae, and A. veronii biovar sobria being the most common human pathogens. 2) Aeromonas bacteria typically cause two main types of infections - gastroenteritis and wound infections, sometimes with bacteremia. Gastroenteritis symptoms include diarrhea, abdominal pain, and vomiting. Wound infections range from mild cellulitis to severe myonecrosis. 3) Identification of Aeromonas involves culturing specimens on selective and general media, followed by biochemical and antimicrobial testing

1. AEROMONAS
BY- SANJU SAH
ST. XAVIER’S COLLEGE, MAITIGHAR, KATHMANDU
DEPARTMENT OF MICROBIOLOGY

2. CLASSIFICATION
• Domain- Bacteria
• Phylum- Proteobacteria
• Class- Gammaproteobacteria
• Order- Aeromonadales
• Family- Aeromonadaceae
• Genus-Aeromonas

3. INTRODUCTION
• Aeromonas is a heterotrophic, facultative anaerobic rod that morphologically resembles member of the
family Enterobacteriaceae.
• Seventeen species of Aeromonas have been described most of which are associated with human
pathogen.
• The most important pathogens are Aeromonas hydrophila, Aeromonas caviae and Aeromonas veroni
biovar sobria.
• The organism are ubiquitous in fresh and brackish water.
• The two major disease associated with Aeromonas are gastroenteritis and wound infections[with or
without bacteremia]

6. HABITAT
• Aeromonas species are indigenous to aquatic environment worldwide.
• Aeromonas have been isolated from fresh water, chlorinated water, polluted water and occasionally
marine water and their numbers are highest at warm months.
• Aeromonas species have also been isolated from stored products and meats and from environmental
and seafood sources (A. Salmonicida).
• Nutritonal requirement is simple and salt supplement is not required for growth; high salt
concentration (6-7%) are inhibitory.
• The organism are associated with a wide variety of diseases and in warm and cold blooded vertebrates
including frogs,fish,reptiles and birds.

7. HISTORY
• In the 1950s Aeromonas spp were reported to be associated with children diarrhoea.
• Because the reports were in German language, it was not until early 1980s that the organism were
recognized as significant causes of gastroenteritis in children by Australian researchers.
• Studies in volunteers in 1985 found no association between Aeromonas and diarrhea in healthy adult
volunteers.
• However, the Aeromonas strains used in the study lack certain important virulence factors, adherence
properties.
• Ailthough no more volunteer studies carried out since then, there is strong epidemiological evidence
that Aeromonas spp are responsible for diarrhea in humans (including traveler´s diarrhea).

8. MORPHOLOGY
• Size- 0.3-1.0µm*1.0-3.5µm.
• Gram negative, straight rods with rounded ends.
• Can grow at temperatures as low as 4ºC (A. salmonicidia).
• Morphologically indistinguishable from other enteric organism such as E. coli.
• Mostly motile by the means of single polar flagellum.

9. VIRULENCE FACTOR
• A number of putative virulence factors have been implicated in the pathogenesis of Aeromonas . These
virulence markers are important to distinguish between potentially pathogenic and non-pathogenic
strains . Aeromonas virulence factors include:
• Toxins
 hemolysins
 cytotoxins
 Enterotoxins
• Invasiveness
• Adherence
• Motility and lateral (polar) flagella:
Recent studies have shown that lateral flagella production by Aeromonas is a pathogenic feature due
to its enhancement of cell adherence ,invasion and biofilm formation.

10. Contd…
• Others:
 Proteases
 Outer membrane proteins
• Recently genes for putative type III secretion system(TTSS) were identified in Aeromonas.
• As shown in other pathogenic gram negative bacteria, the TTSS play an essential role in pathogenicity
as it facilitates the delivery of toxins to host cells.

11. Fig: A. caviae strain showing lateral polar flagella

12. Fig: Unsheated polar flagellum of A. veronii biovar sobria strain CA25

13. PATHOGENESIS
• The presumed route of infection is via oral digestion of contaminated foods or water.
• The acid tolerance response (ATR) gene protects Aeromonas from stomach acid.
• Once enters the GI tract, they compete successfully against normal flora with their elaboration of by products
of metabolism and bacteriocin-like compounds to attach and colonize the lumen of intestine and bowel.
• Aeromonas produces two types of flagella, a constitutively expressed polar flagellum (Pof) and multiple
inducible lateral flagella (Laf).
• POF produces swimmer cells in liquid environments while LAF induces swarming motility on solid medium
surfaces.
• POF play an important role in the initial attachment of bacteria to the GI epithelium, while LAF play an
important role in subsequent processes, including increased cell adherence, biofilm formation, and long-term
colonization.

14. PATHOGENESIS
• Similarly, two distinct types of pili exists in Aeromonas that are related to type I and Pap pili of E.
coli and long wavy type IV pili related to bundle forming unit (BFP) that are significant factors in
intestinal colonization.
• Biofilm development may also br regulated by quorum sensing in Aeromonas.
• Aeromonas species contain luxRI homologs, encoding an acyl-homoserine lactone (acyl-HSL)-
dependent transcriptional activator.
• Mutation in the luxS gene significantly altered biofilm development and enhance virulence but did
not appreciably affect cytotoxic production and TTSS activity.
• Qurum sensing and Lactone production also act in concert with TTSS to regulate the expression of
Aeromonas enterotoxin in the diarrhea production.

15. PATHOGENESIS
• Once established in GI Tract, Aeromonas can apparently produce diarrhea by elaboration of
enterotoxigenic molecules causing enteritis or by invasion of the gastrointestinal epithelium producing
dysentery or colitis.

16. Wound infection
• The initial process involves attachment and colonization.
• Aeromonas species elaborate a wide range of microbial proteases (metalloproteases, serine proteases
and aminopeptidases)
• Degradation of such tissues and proteins can serve as an energy for subsequent multiplication.
• When nutrient sources become depleted, Aeromonas exhibit chemotactic motility in response to
aminoacids, proteins or mucins that trigger rapid migration of Aeromonas into subcutaneous tissues
via motility, leading to colonization of environment with enriched nutrients.
• Many other pathogens also play an important roles in wound infections, including quorum sensing and
TTSS.

18. ASSOCIATED DISEASE
• ASSOCIATED DISEASES — Diarrheal disease is the most common manifestation of Aeromonas infection.The organism has
also been associated with a variety of extraintestinal presentations.
• Diarrhea — Aeromonas spp are associated with a range of diarrheal presentations including:
● Acute, secretory diarrhea, often accompanied by vomiting
● Acute, dysenteric diarrhea with blood and mucus
● Chronic diarrhea, lasting more than 10 days
● Choleric diarrhea with "rice-water" stools
● Traveler's diarrhea (probably the most commonly recognized presentation in the United States)
There have been two reports of A. hydrophila enterocolitis associated with the hemolytic uremic syndrome
(HUS)

19. Contd…
• Wound infections —
 Aeromonas can cause mild to severe wound infections.
 Such wound infections affect men three times more commonly than women.
 The most typical presentation is cellulitis although myonecrosis (with and without gas production),
rhabdomyolysis, and lesions mimicking ecthyma gangrenosum have been reported.
 A case of nearly fatal necrotizing fasciitis from a traumatic leg wound incurred from contact with a
fresh water river highlights the virulence potential of Aeromonads to cause serious disease.
• Bacteremia — Sepsis with Aeromonas species is strongly associated with infection with A. veronii
biovar sobria. These patients present with the classic signs and symptoms of gram-negative sepsis and
may have gastrointestinal symptoms, including abdominal pain, nausea, vomiting, and diarrhease

20. Contd…
• Others :
A wide range of infections that include : meningeal, sore throat, urinary tract, ear, endocarditis,
septicemia.

21. LABORATORY DIAGNOSIS
• Specimen : Stool, blood, CSF, wound, faeces etc.
• Direct microscopy: It reveals gram negative rod.
• ISOLATION:
1. From stool: A loopful of liquid stool specimen is added to 10ml APW and incubated at 37ºC. After
overnight incubation a loopful from APW is plated onto BA and another loopful onto ABA
(Ampicillin BA, 15mg/ L) and incubated at 37ºC overnight.
2. From blood: 10ml of blood is inoculated into blood bottles and incubated at 37ºC for up to 2 weeks.
When growth is observed a loopful of cultures is plated on BA and incubated at 37ºC overnight.

22. Contd…
• From wounds: A wound swab is plated directly on blood agar and on ampicillin blood agar (15mcg/L)
and the swab is then placed in APW. After incubation overnight a loopful from APW is plated on BA
and another loopful onto ABA. Plates are incubated overnight at 37ºC.
• IDENTIFICATION:
 Cultural characteristic:
 ON BA- Wide zone of beta-hemolytic colonies(most strains) but strains of A. caviae are usually non-
hemolytic. Many strains produce characteristics double zones of hemolysis.
 They can grow on mac conkey , XLD, CIN and other routine agar plates.

24. Contd…
• Suspected colonies are tested for oxidase. Oxidase positive are then identified using Biochemical tests:
 Biochemical characteristics:
• Catalase - +ve
• Oxidase - +ve
• Indole - +ve (except A. schubertii)
• MR - -ve
• VP - +ve ( except A. caviae)
• Citrate - -ve
• Production of acid and gas from the fermentation of ;
• Glucose- +ve
• Arabinose- +ve
• Sucrose- +ve
• Mannitol - +ve

25. Contd…
 Antimicrobial testing
• Most Aeromonas strains are resistant to penicillin, ampicillin, carbenicillin, and ticarcillin; most are
susceptible to trimethoprim-sulfamethoxazole (TMP-SMX), fluoroquinolones, second and third
generation cephalosporins, aminoglycosides, carbapenems, chloramphenicol, and tetracyclines

27. TREATMENT
• Aeromonas specied are inherently resistant to β-lactams( chromosomally mediated) and other
antibiotics.
• Drugs of choice includes:
 Fluoroquinolones: ciprofloxacin, Norfloxacin
 Third generation cephalosporins: ceftriaxone, cefuroxime
 Trimehtoprim-dulfamethoxazole (TMP-SMZ)
 Tetracyclines (TE)
• Due to emergence of resistance to TMP-SMZ and TE, antibiotics susceptibility should be performed
for such drugs particularly in developing countries.

28. REFERENCES
1. Clinical and microbiological features of Aeromonas hydrophila-associated diarrhea.
2. Watson, I. M., J. O. Robinson, V. Burke, and M. Gracey. 1985. Invasiveness of Aeromonas spp. in
relation to biotype, virulence factors and clinical features. J. Clin. Microbiol. 22:48–51.
3. Parte, A.C. "Aeromonas". www.bacterio.net.
4. Tille, P.M., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.