Pseudomonas plecoglossicida Virulence Peaks in Cold Water
Ask a pond manager what makes a bacterial outbreak worse and you usually get the same answer. Warm water, heavy organic load, crowded nets. Fair enough. Bacteria grow faster in the heat, so summer feels like the season to worry about.
For one species that instinct is simply wrong. Pseudomonas plecoglossicida virulence peaks at around 18 degrees C, far below the 28 degrees C where the bacterium grows fastest. A 2026 review pulled together the experimental work behind that inversion, and the numbers line up well enough that anyone running ayu, grouper, large yellow croaker, rainbow trout or barramundi should rethink when they watch their stock hardest. It is a spring and autumn problem. It is not a summer problem.
What the organism does
Pseudomonas plecoglossicida is a Gram-negative pathogen of farmed fish that pairs environmental toughness with genuine ability to invade a host. It came out of diseased ayu in Japan in 2000, from cases of bacterial hemorrhagic ascites. Since then it has turned up in large yellow croaker, orange-spotted grouper, hybrid pearl gentian grouper, rainbow trout, pejerrey and barramundi.
The disease it is known for is visceral white spot disease, named after the nodules that show up across the internal organs. Outbreaks are described as explosive. Mortality in hit populations runs above 80 percent, and some epidemiological work puts it at 80 to 100 percent. It now sits on the list of principal pathogens threatening whether aquaculture stays viable.
There is a second life too. The organism has been recovered from human clinical material: cerebrospinal fluid, the lung of a child with pneumonia, pseudomyxoma peritonei microbiota, blood from bacteremia patients. For a production species, an organism that shows up in human samples adds a handling and food safety angle that a purely aquatic pathogen would not carry.
The temperature inversion, in numbers
This is the part worth committing to memory, because it runs against the intuition most of us carry around.
| Condition | Temperature | What happens |
|---|---|---|
| Optimal growth | about 28 C | Fastest replication, low virulence gene expression |
| Pathogenic expression | 18 C | Virulence genes specifically highly expressed |
| Outbreak window | 15 to 20 C | Spring and autumn epidemics |
| T6SS marker Hcp | 20 C | Strongly induced |
| T6SS marker Hcp2 | 30 C | Almost undetectable |
| Extracellular product enzymes | 28 C | Higher activity than at 18 C or 12 C |
Look at that middle pair again. The bacterium multiplies fastest at 28 C and kills best around 18 C. Growth and virulence have come apart. That is unusual enough to be the organism’s defining feature.
The extracellular product data sharpen it further. Amylase and related enzyme activity was significantly higher at 28 C than at 18 C or 12 C. On a plate the warm culture looks busier. In a fish, the cool one is the one that causes disease.
For a farm, this flips the surveillance calendar. The risk window sits at the spring and autumn temperature transitions, not at the summer peak.
The virulence toolkit
The review sorts the organism’s weapons into five groups, and for each one there is experimental evidence of what happens when it is removed.
| Virulence system | Key component | Effect when disrupted |
|---|---|---|
| Type III secretion | ExsA | LD50 rises about 190 fold |
| Type III secretion | PopB / PopD | Adhesion falls from 96.2% to 9.6% |
| Type III secretion | PP_ExoU | PLA1 activity via catalytic dyad S132 to D329 |
| Type VI secretion | T6SS-1 vgrG1a | Host survival rises 40% |
| Type VI secretion | T6SS-1 hcp | Vaccine RPS of 86.3% |
| Flagella | flgC, flgK, flgM | Mortality drops 55% to 80% |
| Flagella | flgM alone | Mortality drops 75% |
| Flagella | fliA (sigma 28) | Mortality drops 30% |
| Flagella | lafK | Grouper survival rises 30% |
| Flagella | FliC | Virulence drops 30% |
| Flagella basal body | flgC, fliP, fliG, fliL | LD50 rises 2.1 to 11.5 fold |
| Outer membrane | mlaE | Virulence attenuated about 30% |
Two rows deserve a second look. First, the adhesion figure. Knock out the PopB and PopD translocators and attachment collapses from 96.2 percent to 9.6 percent. Attachment is step one of infection, and here it hangs almost entirely on a single secretion system.
Second, the type VI secretion system. The organism carries T6SS-1, T6SS-2 and T6SS-3, but only T6SS-1 is specific to fish-pathogenic strains, and low temperature switches it on. That stitches the temperature story and the virulence story into one mechanism instead of two.
Beyond those, the review covers extracellular products, metal nutrient acquisition through ZnuABC, FusA, TonB-ExbB-ExbD and PvdE, and outer membrane barriers including LPS-GalU and the Mla pathway.
The regulatory layers on top
Running virulence genes costs energy, so the organism keeps them on a tight leash. The review describes five overlapping control systems.
Sigma factors set the broad transcription programme: RpoD as housekeeping factor, FliA (sigma 28) for flagella, RpoE as the AlgU homologue, SigX as an ECF factor, RpoN (sigma 54). SigX is specifically highly expressed at 18 C, another thread tying cool water to virulence. RpoE expresses more strongly at 28 C but keeps regulating T6SS-2 throughout.
Two-component systems do the environmental sensing. PvgAS acts as a sodium and temperature switch, GacS/GacA as a global regulator, PhoP/PhoQ responds to magnesium limitation and acidic microenvironments, EnvZ/OmpR handles osmotic pressure.
Quorum sensing runs through a classical LuxI/LuxR acyl-homoserine-lactone system plus a LuxR solo receptor, PplR, with sRNA0024 positively regulating luxR.
Cyclic di-GMP signalling is unusually rich here. The genome encodes 29 GGDEF domain proteins, 24 EAL domain proteins, 15 dual-domain proteins and 1 HD-GYP protein, with YfiR/YfiN and L321_RS15240 named as key nodes.
Small RNAs add a last layer: sRNA0024, sRNA113 and sRNA562, regulating quorum sensing and the flagella and T3SS systems respectively.
The practical read is that there is no master switch. Five layers of control mean the organism can reach the same virulent state by several routes, which is why knocking out a single regulator rarely gives clean attenuation.
What the vaccine work has achieved
The review tabulates relative percent survival from attenuation and vaccine studies. These are not head-to-head, and the underlying trials differ in host species, challenge route and dose, but the range tells you something.
| Vaccine or attenuation strategy | Relative percent survival |
|---|---|
| T6SS-1 hcp deletion | 86.3% |
| Triple deletion (mlaE, ompR, deoR), injection | 80.43% |
| Triple deletion (mlaE, ompR, deoR), immersion | 76.92% |
| gacS deletion live vaccine, immersion | 76.31% |
| OprF plus OprI recombinant adenovirus | 79.07% |
| ompR rpoS double deletion, challenged at 4 weeks | 72.25% |
| ompR rpoS double deletion, challenged at 6 weeks | 64.90% |
| gacS deletion live vaccine, injection | 71.05% |
| Recombinant OprF subunit | 70.60% |
Two things stand out. The T6SS-1 hcp deletion gave the strongest protection at 86.3 percent, which says again how central that system is. And duration matters: the ompR rpoS double deletion fell from 72.25 percent at four weeks to 64.90 percent at six weeks. A vaccine that looks good in a short trial may not hold through a production cycle.
Separately, an ompR rpoS double mutant showed virulence cut by about 107 fold, one of the most strongly attenuated constructs in the whole set.
The assumption worth correcting
The industry tends to treat bacterial virulence as a fixed property of a strain. Identify the organism, look up what it does, treat accordingly.
This organism will not cooperate. Its virulence depends on temperature in a way that runs opposite to its growth rate. Its fish-specific secretion system is switched on by cold. Its sigma factor response differs between 18 C and 28 C. The same bacterium in the same pond is a different threat in April than in August.
The second habit to drop is reading growth rate or enzyme activity as a proxy for danger. Extracellular product enzymes were most active at 28 C. Virulence peaked at 18 C. If you judge risk from how fast a culture grows, you are measuring the wrong thing.
Practical implications for producers and buyers
Move your surveillance to the transitions. If your water passes through 15 to 20 C in spring and autumn, those are the windows where this organism is most likely to blow up. Watch more often, and treat rising mortality during a cooling phase as urgent, not as a seasonal nuisance.
Stop assuming a warm-water problem. The reflex to blame hot weather will send you looking the wrong way during a spring outbreak, and with mortality reported above 80 percent, a delayed diagnosis costs the whole cohort.
Ask suppliers about vaccination, and ask how long protection lasts. Available constructs reach 64.90 to 86.30 percent relative survival, and the one study that measured duration showed it decaying between four and six weeks. A vaccine claim with no duration figure is half an answer.
Take the human isolation reports seriously when handling sick stock. Recovery from cerebrospinal fluid, lung, peritoneal and blood samples means this is not an organism to treat casually at the processing table, even though the review establishes no transmission route.
If you are buying, add temperature history to your questions. A supplier who can tell you the water temperature trajectory during a mortality event is measuring something. One who can only tell you the month is guessing.
Where the evidence is weaker
This is a synthesis of published work, not a new experiment. Every number comes from a separate primary study with its own host species, challenge route, dose and endpoint. That is why the vaccine table is a range and not a ranking.
The mutation effects are laboratory constructs. An flgM deletion cutting mortality by 75 percent is a controlled challenge result, not a field observation, and engineered deletions do not turn up in ponds.
No p values were reported alongside the mortality and adhesion figures in the source material, so the precision of those comparisons cannot be judged from this review alone.
The temperature thresholds come from controlled expression work. Real ponds move around, and a farm swinging between 18 and 22 C within a day may see a different pattern from a tank held steady at 18 C.
Last, the human clinical reports are isolation records. They establish that the organism can be found in human material. They do not establish how often, or by what route, and the review makes no transmission claim.
Frequently asked questions
What is Pseudomonas plecoglossicida?
It is a Gram-negative bacterial pathogen of farmed fish, first isolated in 2000 from diseased ayu in Japan. It causes visceral white spot disease and bacterial hemorrhagic ascites, and outbreaks are described as explosive with mortality above 80 percent. Recorded hosts include ayu, large yellow croaker, orange-spotted grouper, hybrid pearl gentian grouper, rainbow trout, pejerrey and barramundi.
At what temperature is this pathogen most dangerous?
Around 18 degrees C, well below its growth optimum of about 28 degrees C. Virulence genes are specifically highly expressed at 18 C, and outbreaks occur at 15 to 20 C during spring and autumn. The T6SS marker protein Hcp is strongly induced at 20 C and almost undetectable at 30 C. Growth and virulence have come apart, so the cool season is the risk window.
Which fish species are affected?
Ayu was the original host. Since then the organism has been recorded in large yellow croaker, orange-spotted grouper, hybrid pearl gentian grouper, rainbow trout, pejerrey and barramundi. That is a wide host range across freshwater and marine production alike, part of why it counts among the principal pathogens threatening aquaculture sustainability.
How lethal are outbreaks?
Mortality in ascites and visceral white spot outbreaks is reported at over 80 percent, and some epidemiological work puts it at 80 to 100 percent. Outbreaks are described as explosive rather than gradual. In laboratory challenge work, flagella gene knockouts reduced mortality by 55 to 80 percent, and an flgM deletion alone reduced it by 75 percent.
What is visceral white spot disease?
It is the disease most associated with this organism, named for the white nodules that develop across the internal organs of infected fish. A related presentation is bacterial hemorrhagic ascites, the condition seen in the original ayu cases. Both are systemic infections rather than surface lesions, which is part of why they move fast once clinical signs appear.
How does the type III secretion system contribute?
It is central to attachment and to lethal dose. Deleting ExsA raises LD50 by about 190 fold, the largest single attenuation reported. Deleting the PopB and PopD translocators collapses adhesion from 96.2 percent to 9.6 percent. The effector PP_ExoU carries PLA1 activity through a catalytic dyad at S132 and D329.
What makes T6SS-1 different from the other secretion systems?
The organism carries T6SS-1, T6SS-2 and T6SS-3, but only T6SS-1 is specific to fish-pathogenic strains, and low temperature switches it on. That links the cold-water virulence pattern straight to a secretion system. Deleting the T6SS-1 component vgrG1a raises host survival by 40 percent, and a T6SS-1 hcp deletion vaccine reached 86.3 percent relative survival.
Which vaccine approaches have worked best?
A T6SS-1 hcp deletion gave the highest relative percent survival at 86.3 percent. A triple deletion of mlaE, ompR and deoR reached 80.43 percent by injection and 76.92 percent by immersion. A gacS deletion live vaccine gave 71.05 percent by injection and 76.31 percent by immersion. An OprF plus OprI adenovirus construct reached 79.07 percent.
How long does vaccine protection last?
Not as long as most trials measure. The one construct with duration data, an ompR rpoS double deletion, gave 72.25 percent relative survival when challenged at four weeks and 64.90 percent at six weeks. That is a meaningful drop inside a single production cycle, so any vaccine claim should come with a duration figure rather than one peak number.
How is virulence regulated?
Through five overlapping layers. Sigma factors RpoD, FliA, RpoE, SigX and RpoN set broad transcription. Two-component systems PvgAS, GacS/GacA, PhoP/PhoQ and EnvZ/OmpR sense sodium, temperature, magnesium, acidity and osmolarity. Quorum sensing runs through LuxI/LuxR with the solo receptor PplR. Cyclic di-GMP signalling involves 29 GGDEF, 24 EAL, 15 dual-domain and 1 HD-GYP proteins.
Can this pathogen affect people?
It has been recovered from human clinical material, including cerebrospinal fluid, the lung of a child with pneumonia, pseudomyxoma peritonei microbiota and the blood of bacteremia patients, so it is described as an emerging opportunistic human pathogen. The source work records isolations only and makes no claim about transmission route or frequency.
What should a producer change about monitoring?
Shift attention to the spring and autumn temperature transitions. The organism is dangerous at 15 to 20 C, not at summer peaks, so a spike in mortality during a cooling phase deserves escalation rather than a wait-and-see approach. Track water temperature continuously rather than by month. Ask vaccine suppliers for duration data, since protection decays between four and six weeks.
