Clinical Presentation and Outcomes Following Infection With Vibrio spp, Aeromonas spp, Chromobacterium violaceum, and Shewanella spp Water-Associated Organisms in Tropical Australia, 2015–2022

Abstract Background Water-associated bacterial infections cause a wide spectrum of disease. Although many of these infections are typically due to human host commensal Staphylococcal or Streptococcal spp, water exposure can result in infections with environmental gram negatives such as Vibrio spp, Aeromonas spp, Chromobacterium violaceum, and Shewanella spp (collectively VACS). Methods We performed a retrospective analysis of the epidemiology, clinical presentation, and outcomes of deep and superficial infections associated with VACS organisms in our health service between 1 January 2015 and 31 December 2023. Results We identified 317 patient episodes of infection with VACS organisms over this period. Of these, Aeromonas spp (63%) was the most common, followed by Vibrio spp (19%), Shewanella spp (13%), and C violaceum (5%). The majority were isolated from males (74.4%) and involved the lower limb (67.5%). Mild infections were more common than severe presentations, with only 15 (4.7%) admissions to the intensive care unit and 8 (2.5%) deaths. Colonization occurred in 6.9% of patients, in contrast to the perceived severity of some of these bacteria. Copathogens were common and included Staphylococcus aureus (48%) and enteric bacteria (57%). The majority of patients (60%) had no documented water exposure. Initial empiric antimicrobial therapy presumptively covered the susceptibilities of the isolated organisms in 47.3% of patients; however, a lack of VACS-covering empirical therapy was not associated with readmission. Conclusions The isolation of a VACS organism in our setting was often not associated with documented water exposure, which has implications for empiric antimicrobial therapy. Severe disease and death were uncommon.

There is a large spectrum of illnesses attributable to waterassociated bacterial infections.These range from uncomplicated cellulitis to diabetic foot infections, bone and joint infections, rapidly progressive necrotizing fasciitis (NF), and bacteremia.The severity of clinical manifestations depends on the organism and associated virulence factors, inoculum size, mechanism of entry, and patient risk factors.
Although these syndromes, even in the setting of water exposure, are often due to host commensal Streptococcal spp or Staphylococcus aureus [1], water exposure can increase the risk of infection with environmental gram-negative organisms, including Vibrio spp, Aeromonas spp, Chromobacterium violaceum, and Shewanella spp (VACS).These organisms can be found in fresh or saltwater environments and may be associated with contaminants (eg, mud or sewage) or, in some settings, drinking water [2].Importantly, Australian national nonwater-associated empirical antimicrobial guidelines for skin and soft tissue infection (SSTI) do not recommend therapy that will cover these organisms [3].
Pathogenic Vibrio spp are halotolerant and therefore mainly associated with saltwater exposures.Clinical manifestations range from simple SSTI to severe necrotizing syndromes, which are associated with comorbidities such as chronic liver disease, immunocompromised states, iron storage disorders, chronic kidney disease, and diabetes mellitus [4][5][6].Aeromonas spp are typically associated with purulent wound infections or abscesses and are most common in previously healthy individuals with traumatic wounds exposed to freshwater [7].Infection with C violaceum is rare but has been associated with abscess formation and high mortality rates, but findings linking comorbidities and mortality are inconsistent [8][9][10].Shewanella spp infections are uncommon and most often described in relation to chronic wound infections in patients with diabetes.Other associated risk factors for infection include underlying liver disease, malignancy, chronic kidney disease, and immunosuppression [11,12].
There is limited research characterizing the clinical presentations and outcomes of these infections.Further information can assist clinicians treating infections associated with VACS organisms and may help inform empirical antimicrobial guidelines where water exposure occurs.This study aimed to describe the epidemiology, clinical presentation, and clinical outcomes for deep and superficial infections associated with VACS organisms in tropical northern Australia.

METHODS
We conducted a retrospective cohort study to describe the clinical presentation, epidemiology, and outcomes for patients who presented with SSTI, osteoarticular, or bacteremic infections associated with VACS organisms to the Top End Health Service from 1 January 2015 through 31 December 2022.As we sought to investigate infections related to environmental exposure, patients with VACS isolations from other sites (stool, sputum, urine) were excluded.
The Top End Health Service comprises 3 hospitals: the Royal Darwin and Palmerston hospitals (RDPH) in the coastal capital of Darwin, Katherine District Hospital (KDH) in the subtropical interior, and Gove District Hospital (GDH) in the deepwater port of Nhulunbuy, which are spread across ∼500,000 km2 (Supplementary Figure 1).The climate is dominated by the summer monsoon, with interior freshwater floodplains emptying into the Timor Sea or Gulf of Carpentaria over the course of the relatively arid winter.This environment creates many opportunities for occupational or recreational water exposure and subsequently the risk of water-associated infections [4,14].
Cases were identified through the Top End Health Service laboratory system (Labtrak; Intersystems) by searching for all VACS isolates in the study period.Repeat isolations ≤90 days from the index episode were excluded.
Isolates were identified by biochemical and phenotypic methods, including API 20NE (bioMérieux), Vitek 2 (bioMérieux), and/or matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (bioMérieux).Cases with ≥1 VACS organism isolated were aggregated into the more pathogenic species, which we graded in order of decreasing virulence from C violaceum to Vibrio spp, Shewanella spp, and finally Aeromonas spp.
Anonymized data were recorded with the REDCap electronic data capture tool (Yale University).Patient-specific data points captured from the RDPH electronic medical record included admission, demographics, and comorbidities, as well as microbiological, biochemical, clinical, and outcome data.Clinical episode details included the presenting clinical syndrome and several preexisting comorbidities (see supplementary methods for specific definitions).Some isolates were deemed to represent colonization by the attending clinician and therefore not treated.These patients were included to assess for differences in epidemiology or outcomes as compared with isolates judged pathogenic and requiring antimicrobial therapy; for clarity, they are grouped as an additional "infection" syndrome.We were specifically interested in a history of water exposure and type.Patients were grouped by water exposure and salinity (defined as electronic medical record documentation of water exposure, including salinity if specified), no water exposure (documented lack of water exposure), and unknown water exposure (where no documentation of exposure was made).Information on associated injuries or trauma was also collected.
Laboratory markers captured were the initial white cell count (cells × 10 9 /L) and the highest C-reactive protein (milligrams per liter) ≤72 hours from the index microbiology.Organism-specific details recorded include sites of isolation and the isolation of other (non-VACS) organisms.Treatment data were recorded, including initial and subsequent antibiotic therapy and the nature and extent of surgical management.
Clinical outcomes collected were the presence of sepsis or shock (as judged by the admitting clinician), intensive care unit (ICU) admission and length of stay (LoS), infection-specific death, ≤ 90-day all-cause mortality, hospital LoS, and infectionrelated readmission.Infection-related readmission was defined as representation due to either nonresolving local symptoms or new systemic features of infection (ie, fever) within 90 days of the index microbiology.
Given that there are no published Clinical & Laboratory Standards Institute break points for these organisms, we considered Vibrio spp isolates susceptible to ceftazidime, ceftriaxone, ciprofloxacin, and doxycycline; Aeromonas spp to ciprofloxacin, doxycycline, meropenem, piperacillin-tazobactam, and trimethoprim-sulfamethoxazole; C violaceum to ciprofloxacin, doxycycline, meropenem, and trimethoprim-sulfamethoxazole; and Shewanella spp to ceftazidime, ciprofloxacin, gentamicin, meropenem, and trimethoprim-sulfamethoxazole [13].Receipt of an antibiotic effective against the isolated VACS species was considered "VACS-effective therapy."Individual isolate resistance profiles were not matched to patient-specific antimicrobial prescription.We assessed and categorized time frames for prescription of VACS-effective therapy into ≤72 hours, ≤7 days, or any exposure from the date of the index microbiology.We did not stratify by dosing regimen or therapeutic drug levels, only whether the antimicrobials were prescribed or not.
Copathogens were recorded if they were isolated from the same site as the VACS organism.These were grouped into S aureus, Streptococcus spp, enteric flora, pseudomonads, other, or none (supplementary methods).
Statistical analyses were performed with Stata version 17 (StataCorp).Binary and categorical variables were compared via Pearson χ 2 test and continuous variables by the Kruskal-Wallis test.Post hoc analysis of categorical variables with a Fisher exact approach was performed if initial testing met significance criteria (P ≤ .05)as described [15].Tables were created by the Stata module "table1_mc" [16].

Patient Consent Statement
This study was registered with the Human Research Ethics Committee of the Northern Territory Department of Health and Menzies School of Health Research (reference 2023-45), and the requirement for informed consent was waived due to the retrospective and observational study design.

RESULTS
We identified 393 patient episodes where VACS organisms were isolated, 76 of which were excluded for reasons as described in Figure 1, resulting in 317 episodes included in the study, which made up ∼0.7% of all gram-negative organism isolations over the study period.Patient demographic and comorbidity data are shown in Table 1.The most commonly isolated organism was Aeromonas spp (201/317, 63.4%), followed by Vibrio spp (61/317, 19.2%), Shewanella spp (40/317, 12.6%), and C violaceum (15/317, 4.7%).There was a marked male preponderance (236/317, 74.4%), which was similar among VACS groups (P = .12).Indigenous (First Nations) Australians accounted for 119 (38.0%) patients.The majority of infections (243/317, 76.7%) were managed at RDPH, followed by GDH (41/317, 12.9%) and KDH (29/317, 9.1%).Assessment of the interaction between organism and site of isolation was investigated with an assumed null hypothesis of there being no difference in the relative distribution of infections.This was not the case, with post hoc testing finding the rates of Aeromonas spp isolation lower than expected at GDH (P = .003)but higher in KDH (P = .002).Conversely, Vibrio spp were isolated at lower rates from KDH (P = .004)and higher at GDH (P = .02).
Infections typically involved the lower limb, with foot and leg infections accounting for approximately a third of isolations each (110/317 [34.7%] and 104/317 [32.8%], respectively).The proportion of infections of the hand was highest in the Vibrio spp cohort (12/61, 19.7%), whereas infection of the lower limb (foot and leg) was highest in the Shewanella spp group (31/40, 77.5%), though neither reached statistical significance individually (specific body part) or when grouped as upper limb (hand and arm) or lower limb (foot and leg).
Table 4 details the antimicrobial therapy and outcomes of the cohort.Just under half of patients (149/317, 47.0%) received VACS-effective empiric antimicrobial therapy within 72 hours of the index culture.Therapy between 72 hours and 7 days postisolation was VACS effective in just over half of patients (163/ 317, 51.4%), and 47.0% (149/317) received no VACS-effective therapy at any point during the index admission.The receipt of VACS-effective empirical antibiotics was associated with ICU admission (13/15 admissions, 86.7%; P = .002).In total, 41 (13%) of 317 patients were readmitted with a complication of the index infection.A lack of VACS-effective antimicrobial therapy was not associated with readmission, as 23 (56.1%) of 41 patients readmitted had VACS-effective therapy over any time frame (P = .67).
In total, 15 (4.7%) of 317 patients were admitted to the ICU, and 3 (0.9%) died in the hospital due to their infection.Few patients admitted to the ICU (4/15, 26.7%) had documented water exposure.Most ICU admissions were from the Aeromonas spp group (9/15, 60.0%).The majority of clinical syndromes of those admitted to the ICU were superficial SSTI, NF, and bacteremia, all in 4 of 15 patients (26.7%), though only bacteremia and NF were statistically overrepresented (P = .003and P < .001,respectively).Of those admitted to the ICU, 7 (46.7%)presented with shock.A lack of any copathogen was associated with ICU admission (P = .019).For those admitted to the ICU, the median LoS was 4 days (IQR, 2-11) with an overall hospital LoS of 28 days (IQR, 12-55).The median LoS for the overall cohort was 3 days (IQR, 0-7).
The incidence and outcomes of colonizer isolates were compared with the pathogenic group (Supplementary Table 2).Colonizers were associated with higher rates of readmission (P = .038)and 90-day mortality (P = .042)but not with any change in copathogen rates.For colonizers, there were differences in site of isolation (groin overrepresented, P = .013),reduced VACS-effective antibiotic exposure (P = .003),and  overrepresentation of some comorbidities (chronic lung disease, P = .001;heart failure, P = .019;hemodialysis, P = .010;diabetes, P = .014).

DISCUSSION
This study reviews the spectrum of VACS organisms associated with deep and superficial infections from a single tropical health service over an 8-year span.The broadly similar patient demographics, comorbidities, and pattern of clinical syndromes suggest that the previously ascribed acronym "VACS" has merit in terms of VACS-associated infection [13].Although other marine-associated infections are well described (eg, Edwardsiella tarda, Erysipelothrix rhusiopathiae), the VACS group shares several clinical features [2].Other studies have compared some but not all the bacteria, and previous reviews often examined individual species or case reports [9,12,17,18].The marked adult male predominance and few pediatric cases seen in this study were significant and are consistent with findings of previous studies of VACS SSTI [13,18].This male predominance, however, is not a feature of other skin infections, such as community-acquired methicillin-resistant S aureus [18].This may reflect a combination of factors, including exposure-prone occupational and recreational activities and comorbidities such as alcohol use disorder [19].Estrogen is postulated to play a protective role against the effects of V vulnificus endotoxin [20].
Indigenous (First Nations) Australians, especially in central and northern Australia, are often disproportionally affected by infectious diseases.Although there were no species-specific .014 Streptococcus spp Laboratory, c median (IQR) WCC, cells × 10 9 /L 9.9 (8.4-13.5)10.9 (8.8-14.7)12.8 (11.1-14.4)10.9 (7.7-15.3)10.9 (8.6-14.associations with ethnicity, 38% of the cohort identified as Indigenous, which is higher than the Indigenous population rate in our region of 30% [21].This may reflect in part the rural and remote residences of many Indigenous people of the Northern Territory, with close links to the environment in daily activities.The higher proportion of Aeromonas spp from KDH likely reflects its inland geography, while the higher proportion of Vibrio spp at GDH is consistent with the coastal locale, where Indigenous Australians identify as "saltwater people." The levels of chronic diseases seen in this cohort are moderately higher than those of the Northern Territory adult population overall: we found increased rates of chronic kidney disease and hemodialysis (21.8% vs 9.4%), diabetes (14.2% vs 9.1%), and chronic lung disease (4.1% vs 2.9%) in our study vs baseline rates [22].Given the known association between Vibrio spp and Shewanella spp with diabetes and chronic kidney disease, this could have led to an overrepresentation of these organisms; however, Aeromonas spp were still by far the predominant orin our cohort.Although all VACS organisms are known to be water associated, <40% in this cohort had documented water exposure (10.1% had unknown exposure).Aeromonas spp infections are associated with the wet season and can be more ubiquitous in the environment [13,18].However, a considerable number of Vibrio spp and Shewanella spp isolations did not have documented water exposure (18% and 55%, respectively) despite these organisms being strongly associated with salt water [2].
This could be a result of the lack of inquiry or documentation regarding water exposure but could also represent an underrecognized presence of these organisms in nonaquatic environments.Climate change and associated greater water temperatures are projected to increase the distribution of Vibrio spp, markedly raising the populations exposed to this pathogen [23].This highlights the need for clinical consideration of these organisms in not only our tropical environment but other novel settings, particularly with trauma in patients with comorbidities.This study contributes contemporary information about Vibrio spp infections to a wider audience who may be increasingly confronted with such presentations as climate change impacts on microbial ecology.
Due to the previously described associations between VACS organisms and poor outcomes, our institution has produced specific guidelines for empiric treatment of wounds associated with water exposure.Therefore, these guidelines are likely responsible for some of the association between documented water exposure and VACS-effective empirical treatment seen in our study.
For this study, our definition of colonizers was simply those VACS organisms managed as such in the clinical records by the attending clinician.Our data suggest that a potentially even larger number of VACS organisms may have been "colonizers," though not explicitly documented as such.This particularly applies to scenarios where there was coisolation of S aureus and/ or Streptococcus spp and where clinical improvement occurred despite empirical therapy without a VACS-effective antimicrobial.Given that culture results may not be available within the first 48 hours, the decision to switch to VACS-effective therapy on isolation of a VACS organism requires clinical judgment.These high rates of copathogen isolation make it difficult to ascribe the specific virulence or pathogenicity of each isolate.While the isolated VACS may be a colonizer in those already clinically improving, we now recommend consideration of VACS-effective therapy for those with more severe infections or significant comorbidities, who are at higher risk of subsequent readmission and mortality, irrespective of water exposure history.
One group of patients who had adequate levels of VACS-effective empirical therapy, despite little documented water exposure, comprised those admitted to the ICU where increased severity of illness prompts expansion of antimicrobial coverage.This is typically meropenem in our ICU due to the high rates of melioidosis in our region (due to Burkholderia pseudomallei) [24].Of note, B pseudomallei can itself result in water-associated infections [25].Following initial intravenous therapy in the ICU, rationalization of subsequent antimicrobial therapy can be guided by whether the organisms isolated are VACS, B pseudomallei, or others, including communityacquired methicillin-resistant S aureus, which is common in our setting [26].Postoperative wound infections with Aeromonas spp are rare, with only 21 reported cases worldwide until 2009 [27].However, we identified 11 patients with Aeromonas spp from postoperative wound specimens.This is unlikely to reflect exposure to and infection with Aeromonas spp postoperatively but rather prior environmental exposure of a partially healed wound with culture of Aeromonas spp after surgery, usually in an individual with multiple comorbidities.
There are several limitations to this study.As a retrospective study, it relies on the complete and accurate clinical documentation of the treating teams at the time of admission.Considering our results, this limitation is especially relevant to the documentation or not of water exposure.Occult water exposure that is unrecognized by the patient or deemed insufficient to meet an "exposure threshold" by the treating clinician is possible.To better analyze this, prospective studies should ensure adequate recording of water type and specific exposure to allow better assessments of VACS infection risk.Similar issues surround the classification of colonizer VACS isolates, which were documented in only a small proportion of patients.These and other data points relied on the assessment of the treating team, which is made on a case-by-case basis with clinicians possibly applying varied definitions of syndrome, exposure type, or mechanisms of injury.In areas where these infections are more commonly seen, we recommend specific documentation of the presence of water exposure or lack thereof.
Furthermore, due to the lack of published break points and large number of species with each group, we assumed sensitivity of broad classes of antimicrobials to each category of organisms.The large number of copathogen isolations also makes it difficult to precisely determine the specific pathology related to the isolated VACS organism and hence challenging to comment on specific causality.
In summary, VACS group infections have similar withingroup risk factors, clinical manifestations, and outcomes.The isolation of a VACS organism in our setting was often not associated with documented water exposure, and severe disease and death were uncommon.Despite this, we recommend treatment with appropriate antimicrobials should these organisms be isolated.

Figure 1 .
Figure 1.Flowchart describing the number of patients included and excluded from analysis, grouped by species.SSTI, skin and soft tissue infection; VACS, Vibrio spp, Aeromonas spp, Chromobacterium violaceum, and Shewanella spp.
Abbreviations: C violaceum, Chromobacterium violaceum; CRP, C-reactive protein; SSTI, skin and soft tissue infection; VACS, Vibrio spp, Aeromonas spp, Chromobacterium violaceum, and Shewanella spp; WCC, white cell count.a P < .05 in Fisher exact post hoc analysis.b Four patients (2 Aeromonas, 1 Shewanella, and 1 Vibrio) had multiple sites of infection; all sites have been included.c Where recorded.