Infectious Disease and Microbiology – Leuconostoc Species

Overview

Leuconostoc species are Gram-positive cocci that are uncommon causes of human disease. They have a worldwide distribution and may be confused in the laboratory with Enterococcus species or viridans group streptococci.

Although isolation may occasionally represent contamination or colonization, recovery of Leuconostoc from blood cultures should be evaluated carefully, particularly because true bacteremia and infective endocarditis can occur.

A particularly important microbiologic feature is their intrinsic resistance to vancomycin.

Important Species

The source lists:

• Leuconostoc citreum

• Leuconostoc lactis

• Leuconostoc mesenteroides

• Leuconostoc paramesenteroides

• Other Leuconostoc species

Taxonomy within this group has changed over time, so some organisms found in older literature have subsequently been reassigned.

Microbiologic Characteristics

Leuconostoc species are generally:

• Gram-positive cocci

• Facultatively anaerobic rather than strictly anaerobic

• Catalase-negative

• Non-spore-forming

• Lactic acid-producing organisms

Their appearance and biochemical characteristics can lead to confusion with other catalase-negative Gram-positive cocci.

Laboratory Identification

Leuconostoc may be mistaken for:

Enterococcus species

or:

Viridans group streptococci

Accurate identification is clinically important because the antimicrobial susceptibility pattern differs substantially from that of many other Gram-positive cocci.

Vancomycin Resistance

Major High-Yield Feature

The most important antimicrobial characteristic is:

Intrinsic vancomycin resistance

Therefore:

Gram-positive coccus

• ●

Looks like Enterococcus or viridans streptococcus

• ●

Vancomycin resistant

→ Consider Leuconostoc

Clinical Importance of Vancomycin Resistance

Vancomycin is commonly used empirically for serious Gram-positive infections.

However:

Leuconostoc → intrinsically resistant to vancomycin

Thus, failure to identify the organism correctly can lead to inappropriate antimicrobial treatment.

Incubation Period

The incubation period is:

Unknown

A defined incubation period is generally not clinically useful because invasive disease is rare and frequently occurs opportunistically in patients with significant underlying risk factors.

Epidemiology

Leuconostoc species have a:

Worldwide distribution

They are widely encountered in nature and are particularly associated with:

• Plants

• Vegetables

• Fermented foods

• Dairy and food-production environments

Human invasive infection remains uncommon.

Clinical Significance

The source notes that the clinical significance of Leuconostoc species has historically been uncertain.

Isolation may sometimes represent:

Contamination

However, recovery from a normally sterile site—particularly the bloodstream—should not automatically be dismissed.

Bacteremia

Leuconostoc species have occasionally caused:

Bacteremia

The source particularly identifies cases involving:

• Newborns

• Immunocompromised patients

Risk Factors for Invasive Infection

Reported invasive disease is more likely in patients with factors such as:

• Immunosuppression

• Severe underlying illness

• Neonatal age

• Prolonged hospitalization

• Intravascular catheters

• Disrupted gastrointestinal barriers

• Previous broad-spectrum antimicrobial exposure

Because cases are rare, these associations should be interpreted in the overall clinical context.

Blood Culture Interpretation

When Leuconostoc is recovered from blood, consider:

Contaminant?

versus:

True bacteremia/endovascular infection?

Evidence favoring genuine infection includes:

• Multiple positive blood cultures

• Persistent bacteremia

• Fever or sepsis

• Immunocompromised state

• Intravascular device

• Evidence of infective endocarditis

Infective Endocarditis

Leuconostoc species are a:

Rare cause of infective endocarditis

Persistent bloodstream isolation should therefore raise concern for an endovascular focus.

Endocarditis Evaluation

Possible findings include:

• Persistent fever

• Repeated positive blood cultures

• New or changing cardiac murmur

• Valvular vegetation

• Embolic manifestations

When clinically indicated, echocardiography may be necessary to investigate for valvular infection.

High-Yield Endocarditis Pattern

Persistent blood cultures with Leuconostoc

• ●

Fever

• ●

Cardiac/endovascular findings

→ Evaluate for infective endocarditis

Do not automatically dismiss the isolate as contamination.

Diagnosis

The principal diagnostic method is:

Culture

Depending on the infection, specimens may include:

• Blood cultures

• Catheter-associated specimens

• Tissue or other normally sterile material

Correct species identification is particularly important because of the organism’s vancomycin resistance.

Treatment

Penicillin or Ampicillin

The source recommends:

Penicillin G

or:

Ampicillin

For severe infections, the source recommends:

High-dose intravenous therapy

Additional Treatment Options

The source lists:

• First-generation cephalosporins

• Clindamycin

• Imipenem

Because invasive Leuconostoc infection is uncommon, treatment should ideally be guided by:

Antimicrobial susceptibility testing

particularly in severe or endovascular disease.

Avoid Vancomycin

A central treatment principle is:

Do not rely on vancomycin for Leuconostoc.

The organism is:

Intrinsically resistant to vancomycin

This is not simply an occasional acquired resistance pattern; it is a characteristic property of the genus.

Mechanism of Vancomycin Resistance

Vancomycin normally binds to the:

D-Ala-D-Ala

terminus of peptidoglycan precursors.

Leuconostoc uses altered cell-wall precursors ending in:

D-Ala-D-Lac

which greatly reduces vancomycin binding.

This provides the basis for its characteristic intrinsic glycopeptide resistance.

Treatment Principle

For clinically significant Leuconostoc infection:

Confirm true infection

↓

Correctly identify the organism

↓

Recognize intrinsic vancomycin resistance

↓

Perform susceptibility testing

↓

Use an active agent such as penicillin/ampicillin when susceptible

↓

Evaluate persistent bacteremia for an endovascular source

Leuconostoc vs. Enterococcus

Leuconostoc

→ Gram-positive coccus

→ Catalase-negative

→ May resemble Enterococcus

→ Rare opportunistic pathogen

→ Intrinsically vancomycin resistant

Enterococcus

→ Gram-positive coccus

→ Catalase-negative

→ Common GI flora

→ Common cause of UTI, bacteremia and endocarditis

→ Vancomycin susceptibility varies; acquired VRE mechanisms are clinically important

The distinction is particularly important when a presumed “enterococcus” demonstrates unexpected vancomycin resistance.

High-Yield Clinical Pattern

Immunocompromised or neonatal patient

• ●

Gram-positive cocci in blood

• ●

Organism resembles Enterococcus/viridans streptococcus

• ●

Vancomycin resistance

→ Think Leuconostoc

Exam Essentials

Genus: Leuconostoc

Important species: L. citreum, L. lactis, L. mesenteroides, L. paramesenteroides

Morphology: Gram-positive cocci

Metabolism: Facultatively anaerobic

Catalase: Negative

Distribution: Worldwide

Incubation: Unknown

Clinical significance: Usually low pathogenicity but capable of invasive disease

Important hosts: Newborns and immunocompromised patients

Major invasive infection: Bacteremia

Endovascular infection: Rare endocarditis

Diagnostic method: Culture

Laboratory confusion: Enterococcus and viridans streptococci

Source treatment: Penicillin G or ampicillin

Severe disease: High-dose IV therapy described in source

Additional source treatments: First-generation cephalosporin, clindamycin, imipenem

Major antimicrobial clue: Intrinsic vancomycin resistance

Resistance mechanism: Cell-wall precursor ending in D-Ala-D-Lac

Key clinical pearl: The classic clue for Leuconostoc is an unusual catalase-negative Gram-positive coccus that resembles Enterococcus or viridans streptococci but is intrinsically resistant to vancomycin. When repeatedly isolated from blood, particularly in a newborn or immunocompromised patient, it should be taken seriously and persistent bacteremia should prompt consideration of endocarditis.

Important Species The source lists: • Leuconostoc citreum

• Leuconostoc lactis

• Leuconostoc mesenteroides

• Leuconostoc paramesenteroides

• Other Leuconostoc species Taxonomy within this group has changed over time, so some organisms found in older literature have subsequently been reassigned.

Microbiologic Characteristics Leuconostoc species are generally: • Gram-positive cocci

• Facultatively anaerobic rather than strictly anaerobic

• Catalase-negative

• Non-spore-forming

• Lactic acid-producing organisms Their appearance and biochemical characteristics can lead to confusion with other catalase-negative Gram-positive cocci.

Laboratory Identification Leuconostoc may be mistaken for: Enterococcus species or: Viridans group streptococci Accurate identification is clinically important because the antimicrobial susceptibility pattern differs substantially from that of many other Gram-positive cocci.

Vancomycin Resistance Major High-Yield Feature The most important antimicrobial characteristic is: Intrinsic vancomycin resistance Therefore: Gram-positive coccus  ●  Looks like Enterococcus or viridans streptococcus  ●  Vancomycin resistant → Consider Leuconostoc

Clinical Importance of Vancomycin Resistance Vancomycin is commonly used empirically for serious Gram-positive infections. However: Leuconostoc → intrinsically resistant to vancomycin Thus, failure to identify the organism correctly can lead to inappropriate antimicrobial treatment.

Incubation Period The incubation period is: Unknown A defined incubation period is generally not clinically useful because invasive disease is rare and frequently occurs opportunistically in patients with significant underlying risk factors.

Epidemiology Leuconostoc species have a: Worldwide distribution They are widely encountered in nature and are particularly associated with: • Plants

• Vegetables

• Fermented foods

• Dairy and food-production environments Human invasive infection remains uncommon.

Clinical Significance The source notes that the clinical significance of Leuconostoc species has historically been uncertain. Isolation may sometimes represent: Contamination However, recovery from a normally sterile site—particularly the bloodstream—should not automatically be dismissed.

Bacteremia Leuconostoc species have occasionally caused: Bacteremia The source particularly identifies cases involving: • Newborns

• Immunocompromised patients

Risk Factors for Invasive Infection Reported invasive disease is more likely in patients with factors such as: • Immunosuppression

• Severe underlying illness

• Neonatal age

• Prolonged hospitalization

• Intravascular catheters

• Disrupted gastrointestinal barriers

• Previous broad-spectrum antimicrobial exposure Because cases are rare, these associations should be interpreted in the overall clinical context.

Blood Culture Interpretation When Leuconostoc is recovered from blood, consider: Contaminant? versus: True bacteremia/endovascular infection? Evidence favoring genuine infection includes: • Multiple positive blood cultures

• Persistent bacteremia

• Fever or sepsis

• Immunocompromised state

• Intravascular device

• Evidence of infective endocarditis

Infective Endocarditis Leuconostoc species are a: Rare cause of infective endocarditis Persistent bloodstream isolation should therefore raise concern for an endovascular focus.

Endocarditis Evaluation Possible findings include: • Persistent fever

• Repeated positive blood cultures

• New or changing cardiac murmur

• Valvular vegetation

• Embolic manifestations When clinically indicated, echocardiography may be necessary to investigate for valvular infection.

High-Yield Endocarditis Pattern Persistent blood cultures with Leuconostoc  ●  Fever  ●  Cardiac/endovascular findings → Evaluate for infective endocarditis Do not automatically dismiss the isolate as contamination.

Diagnosis The principal diagnostic method is: Culture Depending on the infection, specimens may include: • Blood cultures

• Catheter-associated specimens

• Tissue or other normally sterile material Correct species identification is particularly important because of the organism’s vancomycin resistance.

Treatment Penicillin or Ampicillin The source recommends: Penicillin G or: Ampicillin For severe infections, the source recommends: High-dose intravenous therapy

Additional Treatment Options The source lists: • First-generation cephalosporins

• Clindamycin

• Imipenem Because invasive Leuconostoc infection is uncommon, treatment should ideally be guided by: Antimicrobial susceptibility testing particularly in severe or endovascular disease.

Avoid Vancomycin A central treatment principle is: Do not rely on vancomycin for Leuconostoc. The organism is: Intrinsically resistant to vancomycin This is not simply an occasional acquired resistance pattern; it is a characteristic property of the genus.

Mechanism of Vancomycin Resistance Vancomycin normally binds to the: D-Ala-D-Ala terminus of peptidoglycan precursors. Leuconostoc uses altered cell-wall precursors ending in: D-Ala-D-Lac which greatly reduces vancomycin binding. This provides the basis for its characteristic intrinsic glycopeptide resistance.

Treatment Principle For clinically significant Leuconostoc infection: Confirm true infection ↓ Correctly identify the organism ↓ Recognize intrinsic vancomycin resistance ↓ Perform susceptibility testing ↓ Use an active agent such as penicillin/ampicillin when susceptible ↓ Evaluate persistent bacteremia for an endovascular source

Leuconostoc vs. Enterococcus Leuconostoc → Gram-positive coccus

→ Catalase-negative

→ May resemble Enterococcus

→ Rare opportunistic pathogen

→ Intrinsically vancomycin resistant Enterococcus → Gram-positive coccus

→ Catalase-negative

→ Common GI flora

→ Common cause of UTI, bacteremia and endocarditis

→ Vancomycin susceptibility varies; acquired VRE mechanisms are clinically important The distinction is particularly important when a presumed “enterococcus” demonstrates unexpected vancomycin resistance.

High-Yield Clinical Pattern Immunocompromised or neonatal patient  ●  Gram-positive cocci in blood  ●  Organism resembles Enterococcus/viridans streptococcus  ●  Vancomycin resistance → Think Leuconostoc

Exam Essentials Genus: Leuconostoc

Important species: L. citreum, L. lactis, L. mesenteroides, L. paramesenteroides

Morphology: Gram-positive cocci

Metabolism: Facultatively anaerobic

Catalase: Negative

Distribution: Worldwide

Incubation: Unknown

Clinical significance: Usually low pathogenicity but capable of invasive disease

Important hosts: Newborns and immunocompromised patients

Major invasive infection: Bacteremia

Endovascular infection: Rare endocarditis

Diagnostic method: Culture

Laboratory confusion: Enterococcus and viridans streptococci

Source treatment: Penicillin G or ampicillin

Severe disease: High-dose IV therapy described in source

Additional source treatments: First-generation cephalosporin, clindamycin, imipenem

Major antimicrobial clue: Intrinsic vancomycin resistance

Resistance mechanism: Cell-wall precursor ending in D-Ala-D-Lac

Key clinical pearl: The classic clue for Leuconostoc is an unusual catalase-negative Gram-positive coccus that resembles Enterococcus or viridans streptococci but is intrinsically resistant to vancomycin. When repeatedly isolated from blood, particularly in a newborn or immunocompromised patient, it should be taken seriously and persistent bacteremia should prompt consideration of endocarditis.