Infectious Disease and Microbiology – Weeksella Species

Overview

Weeksella species are rare aerobic Gram-negative bacilli that may occasionally cause opportunistic human infections. The two species traditionally emphasized are Weeksella virosa and Weeksella zoohelcum.

W. virosa has been associated particularly with urinary tract infection and peritoneal dialysis-associated peritonitis, whereas W. zoohelcum has historically been associated with animal exposure and bite-wound infections.

Classification

Genus: Weeksella

Important species:

• Weeksella virosa

• Weeksella zoohelcum

Organism: Aerobic Gram-negative bacillus

Important Taxonomy Note

An important modern taxonomy point is that the organism historically called:

Weeksella zoohelcum

has been reclassified as:

Bergeyella zoohelcum

Therefore, older microbiology references may use:

Weeksella zoohelcum

whereas newer literature generally refers to:

Bergeyella zoohelcum

Microbiologic Characteristics

Weeksella organisms are:

• Gram-negative bacilli

• Aerobic

• Non-spore-forming

• Uncommon human pathogens

W. virosa is generally considered an opportunistic organism and may be encountered in the human genitourinary tract.

High-Yield Microbiology Pattern

Rare Gram-negative bacillus

• ●

Urinary/genitourinary association

• ●

Peritoneal dialysis infection

→ Think WEEKSELLA VIROSA

Incubation Period

A specific incubation period is:

Not clearly established

This is expected because Weeksella generally causes sporadic opportunistic infections rather than a characteristic transmissible syndrome with a predictable incubation period.

Epidemiology

The epidemiology of Weeksella infections remains:

Poorly defined

Human infections are:

Rare

and much of the clinical information comes from isolated cases or small reports.

Weeksella virosa

Weeksella virosa is the principal organism that remains associated with the genus Weeksella in clinical microbiology.

It has been associated with:

• Urinary tract infection

• Genitourinary colonization

• Peritoneal dialysis-associated peritonitis

• Rare opportunistic invasive infections

Urinary Tract Infection

The source identifies:

URINARY INFECTION

as one of the clinical manifestations associated with W. virosa.

Possible symptoms may include:

• Dysuria

• Urinary frequency

• Urgency

• Suprapubic discomfort

Because the organism is uncommon, its recovery from urine should be interpreted together with:

Symptoms + urinalysis + quantitative culture

High-Yield Urinary Pattern

Urinary symptoms

• ●

Unusual aerobic Gram-negative bacillus

• ●

Culture identifies Weeksella virosa

→ Consider true W. virosa UTI

Peritoneal Dialysis-Associated Peritonitis

The source also associates W. virosa with:

PERITONITIS IN PERITONEAL DIALYSIS PATIENTS

Patients may present with:

• Abdominal pain

• Cloudy dialysis effluent

• Fever

• Peritoneal inflammatory findings

The organism may be recovered from:

Peritoneal dialysis fluid

High-Yield Pattern

Peritoneal dialysis

• ●

Abdominal pain/cloudy dialysate

• ●

Rare Gram-negative bacillus

→ Consider Weeksella virosa

Weeksella zoohelcum / Bergeyella zoohelcum

The organism historically known as:

Weeksella zoohelcum

is now generally classified as:

BERGEYELLA ZOOHELCUM

It is associated with:

Animals and animal-associated infections

Animal Bite Wound Infection

The classic infection associated with B. zoohelcum is:

ANIMAL BITE-WOUND INFECTION

Exposure may involve:

Dogs or other animals

with inoculation of the organism into damaged tissue.

High-Yield Animal Exposure Pattern

Animal bite

• ●

Wound infection

• ●

Unusual Gram-negative bacillus

→ Consider Bergeyella zoohelcum

(formerly Weeksella zoohelcum)

Clinical Significance

Because these organisms are rarely isolated, clinicians should consider whether a positive culture represents:

True infection

or

Colonization/contamination

Evidence favoring true infection includes:

• Compatible clinical syndrome

• Isolation from a normally sterile site

• Repeated isolation

• Significant inflammatory response

• Clinical improvement with appropriate treatment

Diagnosis

The primary diagnostic method is:

CULTURE

The organism can be recovered from appropriate clinical specimens such as:

• Urine

• Peritoneal fluid

• Wound material

• Blood in invasive disease

Identification

Because Weeksella is unusual in routine clinical practice, accurate identification may sometimes require:

Modern laboratory identification methods

particularly when conventional biochemical methods provide uncertain results.

Treatment

The source emphasizes that:

ANTIMICROBIAL SUSCEPTIBILITY DATA ARE LIMITED

This is an important point because infections are sufficiently uncommon that extensive clinical treatment data are unavailable.

Penicillin

The source lists:

PENICILLIN

as a treatment option.

However, because susceptibility patterns may vary and these infections are rare, therapy for significant infection should ideally be based on:

Species identification + susceptibility testing + infection site

Additional Treatment

The source also lists:

• Ciprofloxacin

• Trimethoprim–sulfamethoxazole

• Tetracycline

• Aminoglycosides

as possible additional treatments.

These should not be assumed to be universally active because:

Susceptibility may vary between isolates and species.

Source Control

When infection involves a wound or medical device, successful management may require:

Source control

in addition to antimicrobial therapy.

Examples include:

• Proper wound cleaning

• Drainage of infected collections

• Management of infected dialysis equipment when clinically indicated

Weeksella vs. Pasteurella

Both may appear in discussions of unusual Gram-negative organisms, but:

Weeksella virosa

→ Genitourinary association

→ UTI

→ Peritoneal dialysis-associated peritonitis

Pasteurella multocida

→ Strong association with cat and dog bites

→ Rapid cellulitis after animal exposure

Bergeyella zoohelcum vs. Pasteurella multocida

Both may follow:

Animal bites

Bergeyella zoohelcum

→ Rare pathogen

→ Historically called Weeksella zoohelcum

Pasteurella multocida

→ Much more common

→ Classic rapid-onset cellulitis after cat or dog bite

High-Yield Distinction

Animal bite + rapidly developing cellulitis

→ First think Pasteurella multocida

Animal bite + unusual rare Gram-negative organism identified

→ Consider Bergeyella zoohelcum

Weeksella vs. Other Nonfermenting Gram-Negative Bacilli

Rare Gram-negative bacilli may be difficult to distinguish based on morphology alone.

Clinical context can provide important clues:

Urinary/genitourinary or peritoneal dialysis infection

→ Weeksella virosa

Animal bite

→ Bergeyella zoohelcum

Hospital water/device infection

→ Consider organisms such as Pseudomonas or other environmental Gram-negative bacilli

High-Yield Clinical Pattern – W. virosa

Rare aerobic Gram-negative bacillus

• ●

Urinary tract infection

or

Peritoneal dialysis-associated peritonitis

→ Think WEEKSELLA VIROSA

High-Yield Clinical Pattern – Former W. zoohelcum

Animal exposure/bite

• ●

Wound infection

• ●

Rare Gram-negative bacillus

→ Think BERGEYELLA ZOOHELCUM

Exam Essentials

Genus: Weeksella

Important species: W. virosa

Organism: Aerobic Gram-negative bacillus

Epidemiology: Poorly defined; human infection is rare

Major W. virosa infections: UTI and peritoneal dialysis-associated peritonitis

Historical species: W. zoohelcum

Modern name of W. zoohelcum: Bergeyella zoohelcum

Classic B. zoohelcum association: Animal bite-wound infection

Diagnosis: Culture

Treatment evidence: Limited

Source treatment: Penicillin

Other source options: Ciprofloxacin, TMP-SMX, tetracycline, aminoglycoside

Important principle: Use susceptibility-guided therapy for clinically significant infection when possible

Memory Aid

WEEKSELLA VIROSA = WEE-WEE

Think:

WEE

→ Urine

→ UTI

→ Weeksella virosa

And:

ZOOHELCUM = ZOO = ANIMALS

ZOO

→ Animal

→ Animal bite

→ Bergeyella zoohelcum

(formerly Weeksella zoohelcum)

Classic Exam Pattern

Peritoneal dialysis patient

• ●

Peritonitis

• ●

Rare aerobic Gram-negative bacillus

→ Weeksella virosa

OR

Animal bite

• ●

Rare Gram-negative wound pathogen

→ Bergeyella zoohelcum

Key clinical pearl: Weeksella virosa is a rare aerobic Gram-negative bacillus associated particularly with urinary infection and peritoneal dialysis-associated peritonitis. The organism historically called Weeksella zoohelcum, associated with animal bite-wound infections, is now classified as Bergeyella zoohelcum. Because these infections are uncommon and antimicrobial data are limited, clinically significant isolates should be treated according to the infection site and susceptibility results whenever possible.