Persistence of Enterobacter and Citrobacter in Damp Hospital Environments

Hello, and welcome to the ecological foundation of the course. We will approach CPE genomics as a connected problem: bacteria persist in wet hospital niches, genomes and plasmids record some of the mechanisms that make this possible, and genomic comparisons can test—though rarely prove—links between environmental and patient isolates.

This lesson narrows the ecological question to two important groups: Enterobacter, especially the Enterobacter cloacae complex (ECC), and Citrobacter, especially Citrobacter freundii. The central point is that “persistence in a drain” is not a fixed property of a species name. It is an outcome produced by particular lineages, their mobile DNA, the physical niche, and local conditions such as temperature, nutrient supply, flow, disinfectant exposure, and competing microbes.

---

Persistence is an ecological phenotype

A damp hospital environment is not one habitat. A sink drain contains a protected, often nutrient-limited biofilm below the plughole; the sink bowl and rim experience intermittent wetting and cleaning; a toilet has wastewater inputs and flushing disturbance; a shower hose, drain, and surrounding surfaces differ in temperature, flow, and drying. These differences matter because a bacterial lineage that persists in one microhabitat may not be especially successful in another.

For genomic epidemiology, it helps to define persistence precisely. An organism persists when it can remain detectable in a niche over time, either through continuous survival of a resident population or through repeated reintroduction followed by successful regrowth. A single positive swab does not establish persistence.

A wet-site reservoir generally involves four linked biological capacities:

1. Arrival: bacteria enter from wastewater, patient shedding, contaminated hands or equipment, incoming water, or other sources.
2. Attachment and retention: cells adhere to a surface and avoid being fully washed away.
3. Survival and growth: the population tolerates nutrient limitation, variable oxygen, cleaning chemicals, temperature changes, and antimicrobial residues.
4. Release: cells or biofilm fragments disperse during water flow, splashing, flushing, manipulation, or disturbance.

A biofilm is a structured microbial community attached to a surface and embedded in a self-produced matrix. It is not merely “bacteria on a surface.” The matrix can retain moisture and nutrients, change local chemical conditions, and make complete removal difficult. Biofilms also create dense, mixed populations in which bacteria and plasmids repeatedly encounter one another. That can create opportunity for horizontal gene transfer, but opportunity is not evidence that transfer actually occurred.

The schematic is useful as a framework, but it should not be read as proof of a direction of transmission. Recovery of related organisms from a drain and a patient may reflect environment-to-patient transfer, patient-to-environment seeding, or a shared unsampled source.

---

What Enterobacter and Citrobacter have in common

Both genera belong to the order Enterobacterales and include Gram-negative, facultatively anaerobic organisms. Facultative anaerobic means they can grow with oxygen when it is available but can also use alternative metabolic strategies when oxygen is scarce. That flexibility is useful in plumbing biofilms, where oxygen can differ sharply between the biofilm surface, the deeper matrix, and stagnant water.

Both groups can be opportunistic healthcare-associated pathogens. They are particularly relevant where patients are vulnerable because of antimicrobial exposure, invasive devices, surgery, immunosuppression, or prolonged hospital stays. Both also contain lineages that can acquire carbapenemase-bearing plasmids and other mobile resistance elements.

Their shared reservoir-relevant features include:

Feature	Why it can matter in damp hospital sites	What genomics can and cannot show
Flexible metabolism and oxygen tolerance	Supports survival across oxygen-rich surfaces and oxygen-poor drain interiors	Gene content can suggest metabolic capacity, but it does not measure survival in a specific drain.
Surface attachment and biofilm formation	Helps cells remain after water flow and intermittent cleaning	Adhesion or matrix-associated genes are hypotheses, not direct measurements of biofilm biomass.
Antimicrobial resistance	Antimicrobials entering wastewater may select resistant subpopulations under some conditions	Resistance genes and mutations can be detected, but environmental concentrations and actual selection require separate evidence.
Mobile DNA	Plasmids, transposons, and integrons can link resistance traits across bacterial hosts	A plasmid sequence can support shared genetic material; it does not, by itself, date or directionally resolve transfer.
Lineage diversity	Different strains within the same named species may behave differently	Core-genome analysis is needed to distinguish lineage effects from genus-level assumptions.

For the CPE-focused project, an important shared background trait is that clinically important ECC members and the C. freundii complex commonly have chromosomal AmpC-type beta-lactamase capacity. This does not explain every resistant phenotype, and it is distinct from acquiring a carbapenemase gene on a plasmid or transposon. Ecologically, however, a lineage with baseline beta-lactam resistance plus acquired mobile resistance may have a selective advantage in antimicrobial-exposed hospital systems.

The key caution is that resistance, biofilm formation, and patient infection potential do not necessarily travel together. A highly resistant isolate need not be the strongest environmental biofilm former, and a strong biofilm former need not carry the most extensive resistance gene set.

---

Enterobacter: the ECC must be treated as a heterogeneous group

The name Enterobacter cloacae is often used loosely in routine microbiology, but the ECC is a complex of closely related species and lineages, including E. cloacae, E. hormaechei, and E. kobei. This matters immediately for environmental genomics: pooling every ECC isolate into one biological category can hide meaningful differences in resistance, adhesion, nutrient tolerance, and transmission potential.

A useful example comes from a study comparing genetic clusters within the ECC. The authors tested biofilm formation under several culture conditions. Under nutrient-rich media, the clusters did not differ significantly. Under nutrient-deficient minimal medium, however, cluster II—identified as E. kobei—formed stronger biofilms than the other major clusters tested. This is an important ecological pattern: a phenotype may emerge only when experimental conditions resemble a resource-limited niche.

Three deductions from this study are particularly useful for a genomics-centred investigation.

1. Low-nutrient performance can be lineage-specific

Drain biofilms can be nutrient limited, although they are not nutrient free: wastewater, handwashing residues, soaps, skin material, and organic debris can provide episodic inputs. The stronger biofilm phenotype of the E. kobei cluster II strains in minimal medium therefore suggests a testable hypothesis: some ECC lineages may be better equipped to maintain attached populations during periods between nutrient pulses.

It does not establish that every E. kobei isolate persists in every sink, or that a gene found in one isolate causes its success. It does show why species and lineage resolution are necessary before making ecological generalisations.

2. Resistance profiles do not predict environmental persistence by themselves

In the same study, clusters VIII and IX had higher resistance rates to several antimicrobials than cluster II, whereas cluster II showed the stronger minimal-medium biofilm phenotype. The traits did not align into a single “most successful” cluster.

For your future analyses, this means an environmental isolate carrying a carbapenemase plasmid may be retained because of the plasmid, the host chromosome, biofilm-associated traits, local antimicrobial exposure, or a combination of these factors. You should avoid explaining persistence from the carbapenemase alone.

3. Gene presence and phenotype may disagree

The cluster II strains carried fewer of the selected adhesin and siderophore-associated genes, yet they formed the strongest biofilms under nutrient limitation. This is an instructive warning for later gene screening:

• a database may contain only a subset of pathways relevant to attachment and biofilm formation;
• homologous genes can be regulated differently across lineages;
• biofilm formation can arise through multiple biological routes;
• a negative screen is not proof that an isolate cannot attach or persist.

In other words, genomics can identify candidate mechanisms. Phenotype depends on gene content, sequence variation, regulation, environment, and microbial interactions.

---

Citrobacter: temperature and surface zone may be especially important

Citrobacter is also taxonomically diverse, so it should not be reduced to a single environmental phenotype. In this course, C. freundii is especially relevant because it is a frequent healthcare-associated species and can combine chromosomal AmpC biology with acquired resistance determinants.

A particularly useful observation for damp-environment work is that some C. freundii isolates form much stronger biofilms at room temperature than at $37^{\circ }\text{C}$. This contrasts with the common laboratory instinct to assess pathogens mainly at body temperature.

This finding maps plausibly onto the physical structure of damp healthcare environments. The water sitting deep in plumbing may have a different temperature from the sink rim, drain opening, overflow channel, shower surface, or splash zone. A room-temperature phenotype could therefore matter most in the intermittently wet, exposed regions where Citrobacter cells attach at the liquid-air interface.

That does not mean drains are “room temperature” throughout, nor that all Citrobacter form biofilms only at $25^{\circ }\text{C}$. The cited work examined a defined collection of clinical isolates under laboratory conditions. Still, it gives you a more specific ecological hypothesis than the broad statement “Citrobacter forms biofilm”:

Some C. freundii lineages may be particularly capable of maintaining biofilms in cooler, intermittently wet hospital surface zones, while their growth and attachment phenotype may differ at body temperature.

The study also reported aggregative adherence in two multidrug-resistant C. freundii isolates. Yet visible fimbriae were not always observed even when strains adhered strongly. This reinforces the same genomic lesson seen in the ECC comparison: familiar adhesion labels cannot be treated as a complete inventory of attachment mechanisms.

---

A practical comparison for damp-environment genomics

The most defensible comparison is not “Enterobacter persists by one mechanism and Citrobacter by another.” Both can colonise wet niches, form biofilms, carry resistance genes, and act as opportunistic pathogens. The difference is in the evidence currently emphasised for particular lineages and conditions.

Question	ECC-focused interpretation	Citrobacter-focused interpretation
How uniform is the group?	The ECC contains closely related but biologically variable species and clusters; species- and lineage-level analysis is essential.	The genus is also diverse; conclusions from C. freundii should not automatically be assigned to all Citrobacter.
What biofilm condition stands out in the curated evidence?	Cluster II, identified as E. kobei, formed stronger biofilm in nutrient-deficient minimal medium.	Many tested Citrobacter isolates formed stronger biofilm at $25^{\circ }\text{C}$ than at $37^{\circ }\text{C}$.
What ecological niche does that suggest?	Resource-limited, established drain biofilms may favour certain ECC lineages.	Cooler, intermittently wet surfaces and liquid-air interfaces may favour certain C. freundii lineages.
Does resistance explain persistence?	No. Resistance rates varied across ECC clusters and did not simply match the biofilm result.	No. Multidrug resistance was present in studied isolates, but resistance alone did not explain the temperature-dependent phenotype.
What is the genomic implication?	Resolve ECC taxonomy and analyse traits within lineages rather than pooling all “E. cloacae.”	Record species precisely and interpret environmental recovery alongside niche temperature and surface type.
What must remain cautious?	A biofilm-associated gene or lineage association does not prove drain adaptation.	A room-temperature biofilm assay does not prove long-term persistence in hospital plumbing.

From this comparison, you can formulate several testable genomic hypotheses:

• ECC nutrient-persistence hypothesis: repeatedly recovered ECC lineages from drain interiors are enriched for chromosomal or accessory features associated with low-nutrient biofilm formation relative to non-environmental lineages.

• Citrobacter surface-zone hypothesis: C. freundii recovered from cooler sink, shower, or drain-rim niches shows repeated lineage recovery or phenotype-associated genomic variation consistent with attachment under ambient conditions.

• Plasmid-host ecology hypothesis: the same resistance plasmid occurs in different Enterobacter and Citrobacter host lineages because the damp biofilm niche provides repeated co-residence and contact opportunities.

The third hypothesis is deliberately worded carefully. Finding a near-identical plasmid in both genera would support shared mobile DNA, but it would not show whether the transfer happened in a sink, in a patient gut, or in an unsampled upstream reservoir.

---

What to record now, before drawing genomic conclusions

For every environmental isolate, the biological comparison becomes much stronger if the metadata distinguish the microhabitat rather than simply recording “sink” or “bathroom.” At minimum, preserve:

• the organism identification as originally reported, while later confirming it genomically;
• the precise site: drain water, drain biofilm, plughole, sink rim, overflow, toilet water, toilet drain, shower hose, shower drain, or nearby surface;
• date and repeat-sampling history;
• whether the site was wet, recently used, cleaned, or disinfected;
• any available temperature or location proxy, such as room type and water-system context;
• the linked assembly, plasmid calls, and carbapenemase context once those analyses are complete.

This metadata allows genomic patterns to be interpreted as ecological patterns rather than as a list of unrelated sequences. For example, repeated recovery of one ECC lineage from the same drain over months is stronger evidence of local persistence than one environmental isolate carrying a familiar resistance gene. Repeated recovery of a closely related plasmid across different host species is stronger when it occurs in the same place and time, but still requires careful consideration of alternative sources.

---

Key takeaways

Enterobacter and Citrobacter share several traits that make them plausible residents of damp hospital environments: metabolic flexibility, surface attachment, biofilm capacity, antimicrobial resistance, and access to mobile genetic elements. But persistence is conditional, not guaranteed by genus or resistance profile.

Within the ECC, the curated evidence highlights substantial lineage-level variation. In particular, an E. kobei cluster showed stronger biofilm formation under nutrient-deficient conditions despite not having the highest resistance profile or the greatest frequency of selected adhesin-associated genes. For Citrobacter, the key evidence is temperature dependence: tested C. freundii strains formed stronger biofilms at room temperature, a phenotype potentially relevant to cooler, intermittently wet hospital surfaces.

The genomic consequence is simple but essential: resolve hosts beyond broad labels, analyse traits in phylogenetic context, and treat gene detection as evidence of potential rather than proof of environmental behaviour.

Next, we will distinguish clonal strain transmission from the spread of a resistance plasmid—the conceptual distinction needed to decide whether similar CPE from an environmental site and a patient represent the same bacterial lineage, shared mobile DNA, or both.