Infectious Disease and Microbiology – Lactobacillus Species

Overview

Lactobacillus species are Gram-positive, microaerophilic bacilli that normally colonize several areas of the human body, particularly the vagina, oral cavity, and gastrointestinal tract. They are also commonly present in fermented foods and some probiotic preparations.

Although Lactobacillus organisms are usually beneficial commensals with low pathogenic potential, they can occasionally cause serious opportunistic infections such as bacteremia and endocarditis, particularly when host defenses or normal anatomic barriers are disrupted.

Important Species

The source lists:

• Lactobacillus acidophilus

• Lactobacillus casei

• Lactobacillus plantarum

• Lactobacillus rhamnosus

• Lactobacillus salivarius

• Other Lactobacillus species

Taxonomy within this group has undergone substantial revision, so several organisms historically classified as Lactobacillus have subsequently been reassigned to other genera.

Microbiologic Characteristics

Lactobacillus species are generally:

• Gram-positive bacilli

• Non-spore-forming

• Microaerophilic or aerotolerant

• Catalase-negative

• Lactic acid-producing organisms

Their ability to produce lactic acid is especially important in maintaining the normal vaginal environment.

Normal Human Flora

Lactobacillus species commonly colonize the:

Vagina

Oral cavity

Gastrointestinal tract

Therefore, recovery of Lactobacillus from a nonsterile specimen does not necessarily indicate infection.

Vaginal Microbiology

Protective Role of Lactobacilli

In the healthy reproductive-age vagina, lactobacilli contribute to a:

Low vaginal pH

through production of lactic acid.

This acidic environment helps inhibit the proliferation of many potentially pathogenic organisms.

Thus:

Abundant vaginal lactobacilli

→ Lactic acid production

→ Low vaginal pH

→ Suppression of competing organisms

Hormonal Influence

The vaginal Lactobacillus population is influenced by the hormonal environment.

Changes in hormone levels can alter:

• Vaginal epithelial characteristics

• Glycogen availability

• Lactobacillus abundance

• Vaginal pH

• Colonization by other microorganisms

Loss or reduction of protective lactobacilli may permit overgrowth of other bacterial species.

Clinical Significance

A reduction in vaginal lactobacilli can disturb the normal vaginal microbial ecosystem.

This concept is especially important in understanding:

Bacterial vaginosis

in which normal lactobacillus-dominant flora is replaced by a polymicrobial community containing anaerobic and other organisms.

Alterations in vaginal flora may also influence susceptibility to some urinary and genital infections.

Food and Probiotic Association

Lactobacillus species are also found in various:

• Fermented foods

• Dairy products

• Probiotic preparations

Most exposure does not cause disease.

However, in unusual circumstances—particularly in highly vulnerable patients—organisms traditionally regarded as low-virulence commensals can become opportunistic pathogens.

Incubation Period

A conventional incubation period is generally not applicable because most clinically significant infections are:

Endogenous

Infection can develop when organisms belonging to the patient’s normal flora gain access to normally sterile tissues or the bloodstream.

Epidemiology

Lactobacillus species are:

Widely distributed

and commonly present as part of normal human flora.

Invasive infections are relatively uncommon compared with the frequency of colonization.

Bacteremia

Lactobacillus Bacteremia

Lactobacillus species can occasionally cause:

Bacteremia

The source particularly emphasizes bacteremia in:

Neutropenic patients

Other severely immunocompromised or critically ill patients may also be vulnerable to invasive infection.

Interpreting Positive Blood Cultures

Because Lactobacillus is normally associated with human flora, its recovery from blood requires clinical interpretation.

A positive culture should not automatically be dismissed when there are:

• Multiple positive blood cultures

• Persistent fever or sepsis

• Immunosuppression

• Neutropenia

• Evidence of endocarditis

• Another compatible invasive focus

Endocarditis

Lactobacillus species are rare but recognized causes of:

Infective endocarditis

This is one of the most important serious infections associated with the genus.

Clinical Features

Possible manifestations include:

• Persistent fever

• Bacteremia

• New or changing cardiac murmur

• Valvular vegetation

• Embolic complications

• Other manifestations of infective endocarditis

Persistent Lactobacillus bacteremia should therefore raise concern for an endovascular source.

High-Yield Endocarditis Pattern

Persistent Lactobacillus bacteremia

• ●

Compatible cardiac findings

• ●

Valvular vegetation

→ Consider Lactobacillus endocarditis

Do not automatically interpret the organism as insignificant normal flora.

Neonatal Meningitis

The source identifies:

Neonatal meningitis

as a rare invasive manifestation.

Neonates are particularly susceptible to invasive infections because of their relatively immature immune defenses.

Amnionitis

Lactobacillus species have occasionally been associated with:

Amnionitis

or infection involving the amniotic membranes and intrauterine environment.

This represents an uncommon complication because lactobacilli are normally associated with the lower female genital tract rather than invasive disease.

Mediastinitis

Another rare invasive manifestation listed in the source is:

Mediastinitis

This illustrates the ability of normally low-virulence organisms to cause severe disease when introduced into normally sterile tissues.

Diagnosis

The principal diagnostic method is:

Culture

Appropriate specimens depend on the suspected infection and may include:

• Blood cultures

• Cerebrospinal fluid

• Tissue specimens

• Amniotic specimens

• Material obtained from other normally sterile sites

Colonization vs. True Infection

An important clinical principle is distinguishing:

Normal flora/colonization

from:

True invasive infection

Isolation from the vagina, mouth, or gastrointestinal tract is usually compatible with normal colonization.

In contrast:

Repeated isolation from blood or another normally sterile site + compatible clinical disease

provides much stronger evidence of infection.

Treatment

The source lists:

Penicillin G

as the primary treatment.

Additional Treatment

The source lists:

Amoxicillin

as an additional option.

However, clinically important Lactobacillus infections are uncommon, and antimicrobial susceptibility varies among species.

Therefore, serious invasive disease should ideally be managed according to:

Species identification + antimicrobial susceptibility testing + infection site

Important Antimicrobial Consideration

Not all Lactobacillus species have identical antimicrobial susceptibility patterns.

A particularly useful clinical point is that vancomycin cannot be assumed to provide reliable activity against all lactobacilli; some species have intrinsic or substantial resistance.

Thus, treatment of serious infection should not rely solely on the assumption that every Gram-positive organism will respond to vancomycin.

Treatment Principle

For suspected invasive Lactobacillus infection:

Confirm that the isolate represents true infection

↓

Identify the species when possible

↓

Perform susceptibility testing

↓

Look for a source, especially endocarditis in persistent bacteremia

↓

Use an active antimicrobial

The source emphasizes penicillin G or amoxicillin, but definitive therapy should be individualized according to susceptibility and clinical syndrome.

Lactobacillus and Bacterial Vaginosis

A useful microbiologic contrast is:

Normal vaginal flora

→ Lactobacillus-dominant

→ Lactic acid production

→ Low vaginal pH

Bacterial vaginosis

→ Decreased protective lactobacilli

→ Increased polymicrobial anaerobic flora

→ Increased vaginal pH

→ Clue cells may be present

Thus, in the vagina, Lactobacillus is generally protective rather than pathogenic.

High-Yield Clinical Pattern

Gram-positive bacillus

• ●

Normal vaginal/oral/GI flora

• ●

Lactic acid production

• ●

Usually nonpathogenic

• ●

Rare bacteremia or endocarditis in susceptible patients

→ Think Lactobacillus

Exam Essentials

Genus: Lactobacillus

Important species in source: L. acidophilus, L. casei, L. plantarum, L. rhamnosus, L. salivarius

Morphology: Gram-positive bacillus

Oxygen relationship: Microaerophilic/aerotolerant

Major metabolic product: Lactic acid

Normal flora: Vagina, oral cavity, gastrointestinal tract

Food association: Fermented foods and some probiotics

Vaginal function: Helps maintain low vaginal pH

Incubation: Usually not applicable; infection is commonly endogenous

Major invasive infections: Endocarditis and bacteremia

Important bacteremia risk in source: Neutropenia

Other infections: Neonatal meningitis, amnionitis, mediastinitis

Diagnosis: Culture

Source treatment: Penicillin G

Additional source treatment: Amoxicillin

Important treatment principle: Susceptibility varies by species

Vancomycin: Not reliably active against all Lactobacillus species

Key clinical pearl: Lactobacillus species are normally protective Gram-positive members of the vaginal, oral, and gastrointestinal flora, but they can rarely become invasive pathogens. Persistent Lactobacillus bacteremia—especially in a vulnerable patient—should not automatically be dismissed as contamination and should prompt evaluation for a true focus such as infective endocarditis.

Important Species The source lists: • Lactobacillus acidophilus

• Lactobacillus casei

• Lactobacillus plantarum

• Lactobacillus rhamnosus

• Lactobacillus salivarius

• Other Lactobacillus species Taxonomy within this group has undergone substantial revision, so several organisms historically classified as Lactobacillus have subsequently been reassigned to other genera. 

Microbiologic Characteristics Lactobacillus species are generally: • Gram-positive bacilli

• Non-spore-forming

• Microaerophilic or aerotolerant

• Catalase-negative

• Lactic acid-producing organisms Their ability to produce lactic acid is especially important in maintaining the normal vaginal environment. 

Normal Human Flora Lactobacillus species commonly colonize the: Vagina Oral cavity Gastrointestinal tract Therefore, recovery of Lactobacillus from a nonsterile specimen does not necessarily indicate infection. 

Vaginal Microbiology Protective Role of Lactobacilli In the healthy reproductive-age vagina, lactobacilli contribute to a: Low vaginal pH through production of lactic acid. This acidic environment helps inhibit the proliferation of many potentially pathogenic organisms. Thus: Abundant vaginal lactobacilli → Lactic acid production → Low vaginal pH → Suppression of competing organisms 

Hormonal Influence The vaginal Lactobacillus population is influenced by the hormonal environment. Changes in hormone levels can alter: • Vaginal epithelial characteristics

• Glycogen availability

• Lactobacillus abundance

• Vaginal pH

• Colonization by other microorganisms Loss or reduction of protective lactobacilli may permit overgrowth of other bacterial species. 

Clinical Significance A reduction in vaginal lactobacilli can disturb the normal vaginal microbial ecosystem. This concept is especially important in understanding: Bacterial vaginosis in which normal lactobacillus-dominant flora is replaced by a polymicrobial community containing anaerobic and other organisms. Alterations in vaginal flora may also influence susceptibility to some urinary and genital infections. 

Food and Probiotic Association Lactobacillus species are also found in various: • Fermented foods

• Dairy products

• Probiotic preparations Most exposure does not cause disease. However, in unusual circumstances—particularly in highly vulnerable patients—organisms traditionally regarded as low-virulence commensals can become opportunistic pathogens. 

Incubation Period A conventional incubation period is generally not applicable because most clinically significant infections are: Endogenous Infection can develop when organisms belonging to the patient’s normal flora gain access to normally sterile tissues or the bloodstream. 

Epidemiology Lactobacillus species are: Widely distributed and commonly present as part of normal human flora. Invasive infections are relatively uncommon compared with the frequency of colonization. 

Bacteremia Lactobacillus Bacteremia Lactobacillus species can occasionally cause: Bacteremia The source particularly emphasizes bacteremia in: Neutropenic patients Other severely immunocompromised or critically ill patients may also be vulnerable to invasive infection. 

Interpreting Positive Blood Cultures Because Lactobacillus is normally associated with human flora, its recovery from blood requires clinical interpretation. A positive culture should not automatically be dismissed when there are: • Multiple positive blood cultures

• Persistent fever or sepsis

• Immunosuppression

• Neutropenia

• Evidence of endocarditis

• Another compatible invasive focus 

Endocarditis Lactobacillus species are rare but recognized causes of: Infective endocarditis This is one of the most important serious infections associated with the genus. 

Clinical Features Possible manifestations include: • Persistent fever

• Bacteremia

• New or changing cardiac murmur

• Valvular vegetation

• Embolic complications

• Other manifestations of infective endocarditis Persistent Lactobacillus bacteremia should therefore raise concern for an endovascular source. 

High-Yield Endocarditis Pattern Persistent Lactobacillus bacteremia  ●  Compatible cardiac findings  ●  Valvular vegetation → Consider Lactobacillus endocarditis Do not automatically interpret the organism as insignificant normal flora. 

Neonatal Meningitis The source identifies: Neonatal meningitis as a rare invasive manifestation. Neonates are particularly susceptible to invasive infections because of their relatively immature immune defenses. 

Amnionitis Lactobacillus species have occasionally been associated with: Amnionitis or infection involving the amniotic membranes and intrauterine environment. This represents an uncommon complication because lactobacilli are normally associated with the lower female genital tract rather than invasive disease. 

Mediastinitis Another rare invasive manifestation listed in the source is: Mediastinitis This illustrates the ability of normally low-virulence organisms to cause severe disease when introduced into normally sterile tissues. 

Diagnosis The principal diagnostic method is: Culture Appropriate specimens depend on the suspected infection and may include: • Blood cultures

• Cerebrospinal fluid

• Tissue specimens

• Amniotic specimens

• Material obtained from other normally sterile sites 

Colonization vs. True Infection An important clinical principle is distinguishing: Normal flora/colonization from: True invasive infection Isolation from the vagina, mouth, or gastrointestinal tract is usually compatible with normal colonization. In contrast: Repeated isolation from blood or another normally sterile site + compatible clinical disease provides much stronger evidence of infection. 

Treatment The source lists: Penicillin G as the primary treatment. 

Additional Treatment The source lists: Amoxicillin as an additional option. However, clinically important Lactobacillus infections are uncommon, and antimicrobial susceptibility varies among species. Therefore, serious invasive disease should ideally be managed according to: Species identification + antimicrobial susceptibility testing + infection site 

Important Antimicrobial Consideration Not all Lactobacillus species have identical antimicrobial susceptibility patterns. A particularly useful clinical point is that vancomycin cannot be assumed to provide reliable activity against all lactobacilli; some species have intrinsic or substantial resistance. Thus, treatment of serious infection should not rely solely on the assumption that every Gram-positive organism will respond to vancomycin. 

Treatment Principle For suspected invasive Lactobacillus infection: Confirm that the isolate represents true infection ↓ Identify the species when possible ↓ Perform susceptibility testing ↓ Look for a source, especially endocarditis in persistent bacteremia ↓ Use an active antimicrobial The source emphasizes penicillin G or amoxicillin, but definitive therapy should be individualized according to susceptibility and clinical syndrome. 

Lactobacillus and Bacterial Vaginosis A useful microbiologic contrast is: Normal vaginal flora → Lactobacillus-dominant

→ Lactic acid production

→ Low vaginal pH Bacterial vaginosis → Decreased protective lactobacilli

→ Increased polymicrobial anaerobic flora

→ Increased vaginal pH

→ Clue cells may be present Thus, in the vagina, Lactobacillus is generally protective rather than pathogenic. 

High-Yield Clinical Pattern Gram-positive bacillus  ●  Normal vaginal/oral/GI flora  ●  Lactic acid production  ●  Usually nonpathogenic  ●  Rare bacteremia or endocarditis in susceptible patients → Think Lactobacillus 

Exam Essentials Genus: Lactobacillus

Important species in source: L. acidophilus, L. casei, L. plantarum, L. rhamnosus, L. salivarius

Morphology: Gram-positive bacillus

Oxygen relationship: Microaerophilic/aerotolerant

Major metabolic product: Lactic acid

Normal flora: Vagina, oral cavity, gastrointestinal tract

Food association: Fermented foods and some probiotics

Vaginal function: Helps maintain low vaginal pH

Incubation: Usually not applicable; infection is commonly endogenous

Major invasive infections: Endocarditis and bacteremia

Important bacteremia risk in source: Neutropenia

Other infections: Neonatal meningitis, amnionitis, mediastinitis

Diagnosis: Culture

Source treatment: Penicillin G

Additional source treatment: Amoxicillin

Important treatment principle: Susceptibility varies by species

Vancomycin: Not reliably active against all Lactobacillus species 

Key clinical pearl: Lactobacillus species are normally protective Gram-positive members of the vaginal, oral, and gastrointestinal flora, but they can rarely become invasive pathogens. Persistent Lactobacillus bacteremia—especially in a vulnerable patient—should not automatically be dismissed as contamination and should prompt evaluation for a true focus such as infective endocarditis.