This document provides information on the genus Aeromonas. It begins by defining key characteristics, such as being gram negative, oxidase and catalase positive, and facultatively anaerobic. It describes their morphology and discusses the 3 major species implicated in human disease. It then covers the clinical significance of Aeromonas, focusing on gastroenteritis, wound/soft tissue infections, sepsis, and other miscellaneous infections. The document concludes by describing laboratory techniques for diagnosis, including use of selective media and biochemical testing.
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Aeromonas Postgraduate Seminar Maulana Azad Medical College Delhi
1.
2. Definition (Topley Wilson 10th Ed)
Gram negative
Oxidase +
Catalase +
Facultative anaerobic rods
All are motile by means of a single polar flagellum
Except: A. salmonicida & A. media : Non motile
Nitrate reduction +
Indole production +
Except: A. schubertii : Indole -ve
High salt conc. (6-7%) are inhibitory
3. Further Description
(ASM Manual 10th Ed 2011)
They are chemoorganotrophic, displaying
oxidative and fermentative metabolism of
glucose.
Acid, and often acid with gas, is produced
from many carbohydrates, especially glucose
A variety of exoenzymes are produced
Arylamidases esterases
Amylase peptidases,
DNase, proteases,
chitinase, chondroitinase,
hemolysins
4. Classification & Taxonomy
In mid 1970s the genus Aeromonas was divided into 2
groups:
PSYCHROPHILIC: only one spp – A. salmonicida , a
fish pathogen. It is non motile & doesn’t grow at 37° C.
MESOPHILIC: all the other identified spp.
Mesophilic: A mesophile is an organism that grows best
in moderate temperature, neither too hot nor too cold,
typically between 25 and 40 °C (77 and 104 °F).
Psychrophilic: Psychrophiles or cryophiles are
extremophilic organisms that are capable of growth and
reproduction in cold temperatures, ranging from −15°C
to +10°C.
5. Classification & Taxonomy…contd
Order: Aeromonadales
Family: Aeromonadaceae
Other members of the Order: Oceanimonas and
Tolumonas
Aeromonas is the only one of these three genera
that is pathogenic for humans.
6. Organism
Human isolation
(extraintestinal/fecal)
Human pathogen
(extraintestinal/fecal)
Frequency in humans
Pathogenic for
animals,
fish, and reptiles
A. hydrophila complex
A. hydrophila
subsp. hydrophila Yes Yes Common Yes
subsp. dhakensis b Yes Yes Rare No
subsp. ranae No No — Yes
A. bestiarum No/yes —/no Rare Yes
A. salmonicida c No/yes No/no Rare
subsp. salmonicida Yes
subsp.
achromogenes
Yes
subsp. masoucida Yes
subsp. smithia Yes
subsp.
pectinolytica
No
Members of the genus Aeromonas
7. Organism
Human isolation
(extraintestinal/fecal)
Human pathogen
(extraintestinal/fecal)
Frequency in humans
Pathogenic for
animals,
fish, and reptiles
A. hydrophila complex
A. hydrophila
subsp. hydrophila Yes Yes Common Yes
subsp. dhakensis b Yes Yes Rare No
subsp. ranae No No — Yes
A. bestiarum No/yes —/no Rare Yes
A. salmonicida c No/yes No/no Rare
subsp. salmonicida Yes
subsp.
achromogenes
Yes
subsp. masoucida Yes
subsp. smithia Yes
subsp. pectinolytica No
Members of the genus Aeromonas
9. Organism
Human isolation
(extraintestinal/fecal)
Human pathogen
(extraintestinal/fecal)
Frequency in humans
Pathogenic for animals,
fish, and reptiles
A. veronii complex
A. veronii bv. sobria Yes Yes Common Yes
A. veronii bv.
veronii
Yes Yes Rare No
A. jandaei Yes Yes/unknown Rare No
A. trota Yes Neither Rare No
A. schubertii Yes/no Yes/— Rare No
A. encheleia Yes/no No/— One case No
A.
allosaccharophila
No/yes —/no Very rare Yes
A. sobria Neither — — No
A. popoffii Yes Yes Very rare No
A. culicicola No No — No
A. simiae No No — No
A. molluscorum No No — No
A. bivalvium No No — No
A. tecta Yes No Rare No
A. piscicola No No — Yes
10. Organism
Human isolation
(extraintestinal/fecal)
Human pathogen
(extraintestinal/fecal)
Frequency in humans
Pathogenic for animals,
fish, and reptiles
A. veronii complex
A. veronii bv. sobria Yes Yes Common Yes
A. veronii bv. veronii Yes Yes Rare No
A. jandaei Yes Yes/unknown Rare No
A. trota Yes Neither Rare No
A. schubertii Yes/no Yes/— Rare No
A. encheleia Yes/no No/— One case No
A. allosaccharophila No/yes —/no Very rare Yes
A. sobria Neither — — No
A. popoffii Yes Yes Very rare No
A. culicicola No No — No
A. simiae No No — No
A. molluscorum No No — No
A. bivalvium No No — No
A. tecta Yes No Rare No
A. piscicola No No — Yes
11. Straight, coccobacillary to bacillary cells with
rounded ends
0.3 to 1.0 μm in diameter and 1.0 to 3.5 μm in
length.
They can occur singly, in pairs, or, rarely, in short
chains.
Most species are motile by a single, polar
flagellum with a 1.7-μm wavelength, but
peritrichous flagella may be formed on solid
media in young cultures and lateral flagella
occur in some species.
Morphology and Cell Structures
13. The type strain Aeromonas hydrophila subsp.
hydrophila ATCC 7966 was the first aeromonad
to be completely sequenced, annotated, published,
and deposited in GenBank (as CP000462)
This was followed just recently by the publication
of the complete genome sequence of Aeromonas
salmonicida subsp. salmonicida A449, an agent of
furunculosis (a bacterial septicemia of salmonid
fish), which was deposited in GenBank as NC
00938
Type strain
14. The spectrum of human infections caused by
Aeromonas spp usually fall into the 4 broad
categories:
Gastroenteritis
Cellulitis and wound infection
Septicemia
Miscellanous
Clinical significance
15. There are currently 20 named species
but only 3 are of major clinical importance:
A. hydrophila
A. caviae
A. veronii biovar sobria
Clinical significance
16. Aeromonas was first isolated more than 60
years ago, but evidence implicating this genus
as a cause of gastrointestinal disease has
been amassed only since the early 1980s
Aeromonas caviae is the predominant isolate
from diarrheal stools, but in some geographic
areas, A. hydrophila and A. veronii biovar
sobria are frequently isolated as well
Gastroenteritis
17. Evidence supporting a causative role in diarrheal
disease includes:
A higher carriage rate in symptomatic compared
with asymptomatic individuals;
An absence of other enteric pathogens in most
symptomatic patients harboring Aeromonas species
Identification of Aeromonas enterotoxins (although
the absence of an animal model has hampered
efforts to directly link toxin production with
disease)
Gastroenteritis …contd
18. Evidence supporting a causative role in diarrheal
disease also includes:
Improvement of diarrhea with antibiotics
active against Aeromonas species and clinical
worsening with antibiotics ineffective against
the organism; and
Evidence of a specific secretory immune
response (IgA) coincident with diarrheal
disease.
Gastroenteritis …contd
19. Diarrhea is usually watery and
self-limited
But some persons develop fever,
abdominal pain, and bloody stools.
Fecal leukocytes may be present.
Gastroenteritis …contd
20. Occasionally, diarrhea may be severe or
protracted, and hospitalization may be
necessary.
Chronic colitis following acute Aeromonas-
associated diarrhea has been reported in
adults.
Hemolytic uremic syndrome associated with
Aeromonas enterocolitis has been described in
infants and adults
Gastroenteritis …contd
21. Most Aeromonas soft tissue infections are
caused by A. hydrophila.
Trauma followed by exposure to fresh water
(and not salt water, even though aeromonad
density in seawater is similar to that in fresh
water) usually, but not invariably, precedes
infection.
Cellulitis develops within 8 to 48 hours, and
systemic signs are common.
Cellulitis & wound infection
22. Suppuration and necrosis around the
wound are frequent, and surgical
débridement is often necessary.
Fasciitis, myonecrosis (occasionally
associated with gas formation), and
osteomyelitis may develop.
Cellulitis & wound infection …contd
23. Aeromonas soft tissue infections can
develop
after exposure to soil
in association with crush injuries, and
as a complication of burns, typically
when initial management of the burn
included immersion in natural water
sources.
Cellulitis & wound infection …contd
24. Cellulitis & wound infection …contd
There is one reported outbreak of A.
hydrophila wound infections in participants of
a mud football competition in Australia. The
field was “prepared” with water from an
adjacent river.
25.
26. In relation to Hirudotherapy
Aeromonas soft tissue infection is a recognized
complication of the use of medicinal leeches in
conjunction with reimplantation or flap surgery.
27. In relation to Hirudotherapy
Aeromonas hydrophila and other Aeromonas species
are normal inhabitants of the foregut of leeches.
Leeches lack the requisite proteolytic enzymes
and are dependent on the symbiotic Aeromonas to
digest the blood meal.
Aeromonas infection has developed in 7% to 20% of
patients treated with leeches.
Prophylactic antibiotics now have been
recommended at the time of leech application.
Mild wound infection, loss of flap,myonecrosis,
and sepsis may ensue
28. The mortality rate for Aeromonas sepsis is 30%
to 50%.
Taiwan Study: till date the biggest
Cochrane evidence:
Aeromonas bacteremia and sepsis are uncommon,
but in the largest series reported to date, 143
Aeromonas bacteremias, including 104 that
were monomicrobial, occurred in one institution in
Taiwan over a 10-year period.
Septicaemia
29. Taiwan Study…contd
Aeromonas hydrophila caused 60% of the
bacteremias
most of the other isolates that were identified
by species were A. veronii subtype sobria and
A. caviae.
Most patients in this series were
immunocompromised, including 54% who were
cirrhotic and 21% who had an underlying
malignancy.
Septicaemia
30. The US Experience:
There was a similar distribution of Aeromonas
species in a study of 53 Aeromonas blood
isolates collected from 27 medical centers in
the United States over a 10-year period.
Most patients were immunocompromised, and
underlying malignancy was much more common
than liver disease in this series. Most patients
with Aeromonas sepsis do not present with
diarrhea. Interestingly, about one third of
Aeromonas bacteremias are nosocomial.
Septicaemia
31. The US Experience …contd:
Aeromonas has been recovered from hospital
water supplies, and clusters of nosocomial
Aeromonas bacteremia have been described.
However, in one study in which molecular typing
was performed, many different genotypes were
found.
So, mostly the nosocomial cases were not
epidemiologically linked, and endogenous gut
flora was the presumed source.
Septicaemia
32. A variety of other infections caused by Aeromonas
species have been reported, including:
intra-abdominal abscess,
pancreatic abscess,
hepatobiliary infection,
spontaneous bacterial peritonitis in patients with
cirrhosis,
Meningitis
Miscellaneous Infections
34. A. hydrophila epididymitis and bacteremia
developed in a healthy man
Before 24 hours he had sexual intercourse
with his wife
in their swimming pool
Cultures obtained from the pool grew A.
Hydrophila.
Ref: Blair JE, Woo-Ming MA, McGuire PK: Aeromonas hydrophila
bacteremia acquired from an infected swimming pool. Clin Infect
Dis 1999; 28:1336-1337.
Miscellaneous Infections
35. Laboratory diagnosis
Selective Media for Id from Faeces:
Blood Agar (+/- Ampicillin) 10µg/mL added
Alk Peptone Water (pH 8.6) : overnight
enrichment recommended, then S/C
CIN Agar - originally developed for
Yersinia enterocolitica
Enteric Agars:
Deoxycholate
MAC
XLD Agar
36. Laboratory diagnosis …contd
Modified cefsulodin-Irgasan-novobiocin
(CIN) (4 μg/mL of cefsulodin versus 15
μg/ml in unmodified CIN) is also an
excellent isolation medium for aeromonads.
On this medium, Aeromonas colonies have a
pink center with an uneven, clear
apron and are indistinguishable
from Yersinia enterocolitica
morphologically.
37. Laboratory diagnosis
Oxidase + Thus quickly excluded from
Enterobacteriaceae
Motile
Indole + This excludes Pseudomonas
OF test: Aeromonads utilize glucose
fermentatively, thus excluded from
Pseudomonas again
38. Laboratory diagnosis
most clinically relevant species are beta-
hemolytic, including an increasing number
of A. caviae strains, beta-hemolytic
colonies on blood agar should be screened
with oxidase and a spot indole test
Plesiomonas is easily differentiated from
Aeromonas by positive reactions in
Moeller’s lysine, ornithine, and arginine tests
and by fermentation of m-inositol
39. Aeromonas in Blood Agar
Ref: http://www.microbiologyatlas.kvl.dk
Medium sized to large, smooth
colonies, which are white, or
especially in older cultures, buff in
colour. The colonies have an entire
margin.
The same Blood Agar plate examined
with transmitted light. The colonies
are surrounded by a wide haemolysis
zone.
40. Laboratory diagnosis
6.5% NaCl: Distinguishes from Vibrio
fluvialis causing similar infections
Resistance to vibriostatic agent O/129 :
150 μg of the vibriostatic agent 2,4-
diamino-6,7-diisopropylpteridine
differentiates from all other
Vibrios
43. Biochemical identification of Aeromonas
to complex level
No. of strains identified as belonging to
Test
A. hydrophila
complex
(A. hydrophila, A.
bestiarum,
A. salmonicida)
A. caviae
complex
(A. caviae,
A. media, A.
eucrenophila)
A. veronii
complex
(A. veronii HG8, bA.
jandaei,
A. schubertii, A.
trota)
Esculin 87 (92, 81, 85) 71 (76, 55, 78) 0
Voges-Proskauer 75 (88, 63, 62) 0 54 (88, 87, 17, 0)
Glucose (gas) 81 (92, 69, 77) 16 (0, 0, 78) 87 (92, 100, 0, 69)
L-Arabinose 93 (84, 100, 100) 96 (100, 100, 78) 4 (12, 0, 0, 0)
44. Exception Factsheet
All are Motile except:
A. salmonicida
A. media
All are Indole + except:
A. schubertii
All are Arg decarb + Ornith -ve except:
A. veronii biovar veronii
All are VP + Lysine decarb +ve except:
A. caviae complex: VP –ve Lysine decarb -ve
45. The clinically relevant Aeromonas spp are uniformly
resistant to penicillin and ampicillin
Often resistant to cefazolin and ticarcillin
Usually but not invariably susceptible to third-
generation cephalosporins, aztreonam, and
carbapenems.
Resistance to cefotaxime has developed on therapy.
Sensitivity to piperacillin and ticarcillin-clavulanate is
variable.
Antimicrobial Susceptibility and
resistance patterns
46. Aeromonas spp produce as many as three β-
lactamases,
1. A Bush group 2d penicillinase
2. A group 1 cephalosporinase
3. and a metallocarbapenemase
Some isolates exhibit coordinated expression of
these β-lactamases after both induction and
selection of derepressed mutants.
Antimicrobial Susceptibility and
resistance patterns
47. Despite the presence of a carbapenemase
imipenem MIC typically remain low,
although A. jandaei and A. veronii subtype
veronii can display imipenem resistance.
Unlike most carbapenemases, the
Aeromonas metallocarbapenemases have
narrow substrate profiles and specifically
hydrolyze carbapenems.
Antimicrobial Susceptibility and
resistance patterns
48. There are reports of increasing resistance to
tetracycline and trimethoprim-sulfamethoxazole.
In one report, tigecycline was active against
200 of 201 isolates.
Aminoglycosides are usually active, with
resistance to tobramycin being more common
than resistance to gentamicin or amikacin.
Antimicrobial Susceptibility and
resistance patterns
49. Fluoroquinolones are highly active against
Aeromonas spp, although the existence of
nalidixic acid–resistant strains containing
mutations in the gyrA gene raise concern that
fluoroquinolone resistance could easily develop.
Aeromonas species harboring a conjugative
plasmid that confers multiple antibiotic
resistance have been identified.
Antimicrobial Susceptibility and
resistance patterns
50. Antimicrobial Susceptibility and
resistance patterns
Susceptibility a Antibiotic agent
Resistant Ampicillin (except A. trota [100% susceptible]), A. caviae [35% susceptible]
Variable
Ticarcillin or piperacillin (except A. veronii bv. veronii [100% resistant],
A.trota [100% suscep-tible])
Cephalothin
Cefazolin
Cefoxitin (except A. veronii bv. veronii [100% susceptible])
Cefuroxime
Ceftriaxone
Cefotaxime
Susceptible Ciprofloxacin c
Gentamicin
Amikacin
Tobramycin (A. veronii bv. veronii [42% resistant])
Imipenem (A. jandaei [65% resistant], A. veronii bv. veronii [67% resistant])
Trimethoprim-sulfamethoxazole
Dhakensis in Dhaka, controversy to A. aquarorium
In Red: Human Pathogens
Contd in next slide Antibiotic and IgA
Here we see the complicated course of Gastroenteritis
See ref 7 for chronic colitis
Ref 8 for HUS
In fact this is a benign picture where boys are playing on naturally water logged playgrounds. In mud football competition the field is artificially water logged and the view is also rather enjoyable, just with a little Aeromonas
use of medicinal leeches Hirudo medicinalis in conjunction with reimplantation or flap surgery. These leeches are applied to reduce congestion of blood and improve graft uptake
Also recommended to apply leech only in areas with good arterial blood supply. So that infection to necrotic part by Aeromonas doesn’t occur
Also continued in next slide
Also continued in next slide
1 or 2 drops of oxidase reagent T-Meth-Para-Phen-Diamine-DiHcl on colonies Characteristics Black Discolouration
Aeromonas spp. are most easily confused in the laboratory with other oxidase-positive fermenters, i.e., Vibrio and Plesiomonas spp.
Aeromonas spp. are most easily confused in the laboratory with other oxidase-positive fermenters, i.e., Vibrio and Plesiomonas spp.