Yeah, it's late. Tough. What with moving house over the weekend, having learnt that I had to move on Monday (long story), I've been a little preoccupied. I rather doubt that anyone marks their calendar for these posts anywho.
Anyway, Aeromonas is a genus of rod-shaped facultatively anaerobic gamma proteobacteria that is abundant in aquatic habitats (mostly freshwater). 16S rDNA phylogenetically speaking, it belongs to a clade that also includes the Vibrionaceae, Pasteurellaceae and Enterobacteriaceae, all of them pretty economically significant families because they all contain common pathogens, and Aeromonas follows suit. Of the about fifteen species currently recognised in the genus, at least five have been implicated in human infections, while the others are known primarily as pathogens of aquatic organisms such as fish and frogs. The most common symptoms of Aeromonas infection are gastrointestinal diseases (just to make you wince, a quote from Janda & Abbott, 1998 - "Chronic diarrhea exceeding 1 year's duration due to [Aeromonas] caviae or A. hydrophila has been recorded"), but conditions such as septicaemia, wound infections or even meningitis have been caused by this genus, among others.
So far, Aeromonas species have been almost entirely studied as pathogenic and/or animal-associated organisms. However, they have been isolated in large numbers from freshwater and brackish habitats (particularly nutrient-rich ones) (Kaper et al., 1981; Martin-Carnahan & Joseph, 2005), and, in comparison with other genera such as Streptococcus, Staphylococcus and Vibrio, appears to me to show every sign of being primarily a facultative pathogen. Infection causes a range of pathologies, with no obvious standard symptom arc*, low host specificity, and without a tight correlation between the presence of Aeromonas and whether or not a pathology develops (Aeromonas has been isolated from the gut of patients with no sign of Aeromonas-related pathologies). Compare this to an obligate pathogen such as Treponema pallidum (the cause of syphilis), which causes a more standardised range of symptoms and for which presence of the pathogen almost always leads to presence of the disease**. It is quite possible that Aeromonas' role in the environment as a pathogen is in fact only secondary to whatever else it's doing out there (my first guess would be decomposition). However, this neglect of Aeromonas' environmental ecology is hardly unusual. The truth is that our knowledge of environmental real-time ecology for all bacteria is depressingly low. Not only have pathogens received the greater share of attention, but even studies of non-pathogenic taxa have generally been conducted in highly artificial laboratory environments (for obvious reasons of practicality). Laboratory culture media, for instance, generally offer a uniform abundance of nutrients that is entirely unlike the poorer and more patchy nutrient distributions elsewhere, and it is a very open (and very difficult) question how much and in what way observations made in the laboratory environment really reflect what happens elsewhere.
*Yeah, you can tell I'm not thoroughly briefed in my medical terminology and I'm just making this up as I go along, can't you?
**Anybody out there with actual medical knowledge can feel free to rip into me if I've just made an idiot of myself.
Back to Aeromonas. While (as I said) about fifteen or so species are currently recognised, traditionally these have been divided into two groups - Aeromonas salmonicida (guess what it is known to do) vs. everything else. Aeromonas salmonicida is psychrophilic (it grows best at low temperatures, from 2 to 30°C) and nonmotile, while the other species (the 'Aeromonas hydrophila' group) are mesophilic (growing at temperatures between 10 and 42°C) and motile, usually with a single polar (at one end) flagellum but occasionally with lateral flagella. 16S rDNA phylogeny, however, shows that A. salmonicida is nested deep within the genus, and represents a derived taxon. Reflecting this position, A. salmonicida still possesses the genes for flagella production despite their inactivity. And just to really hammer the point home, an as-yet unnamed Aeromonas strain has been isolated (Martin-Carnahan & Joseph, 2005) that is mesophilic, motile, but genetically identical to A. salmonicida!
Janda, J. M., & S. L. Abbott. 1998. Evolving concepts regarding the genus Aeromonas: an expanding panorama of species, disease presentations, and unanswered questions. Clinical Infectious Diseases 27: 332-344.
Kaper, J. B., H. Lockman, R. R. Colwell & S. W. Joseph. 1981. Aeromonas hydrophila: ecology and toxigenicity of isolates from an estuary. Journal of Applied Microbiology 50 (2): 359-377.
Martin-Carnahan, A., & S. W. Joseph. 2005. Genus I. Aeromonas Stanier 1943, 213AL. In G. M. Garrity, D. J. Brenner, N. R. Krieg & J. T. Staley (eds) Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 2. The Proteobacteria. Part B. The Gammaproteobacteria, pp. 557-578. Springer.