Clinical Microbiology Newsletter
Lactobacillus: a Review

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Abstract

Lactobacilli colonize the gastrointestinal and urinary tracts of humans, making them an integral part of the microbial flora. However, in certain circumstances, they can cause disease. Although lactobacilli are often misidentified as streptococci, it is highly likely that these infections will be attributed to them due to current improvements in laboratory techniques. A recent resurgence in interesting natural products has caused an increased focus on the use of probiotics. Many of these probiotic formulations contain Lactobacillus species. Although there have been reports of lactobacilli providing benefit in infectious diarrhea, the use of these probiotic products in immunosuppressed or critically ill patients is not advised, since these populations are at increased risk of developing infections due to lactobacilli.

Introduction

Lactobacilli are non-spore-forming, gram-positive rods that are an important part of the normal human bacterial flora commonly found in the mouth, gastrointestinal (GI) tract and female genitourinary tract (1, 2, 3). Microscopically, these bacteria appear as non-motile, thin rods varying in length from long to short. They can also appear as coryneform with a bent morphology or tend to grow in chains. Most species of lactobacilli are facultative anaerobes growing in either the presence or absence of an anaerobic environment. Only about 20% of species isolated from humans are obligate anaerobes. Lactobacilli tend to grow well on blood agar (4). Some, but not all, species will also grow on Lactobacillus selective medium. Lactic acid is the major metabolic end product of lactobacilli during glucose fermentation (2, 3). Acetic and succinic acids are also produced, but only in small amounts.

The GI tracts of various mammals are commonly colonized with Lactobacillus spp. (2, 3). The most common species of lactobacilli isolated from GI tracts are Lactobacillus brevis, L. casei, L. acidophilus, L. plantarum, L. fermentum, and L. salivarius. Lactobacilli have also been isolated from tooth plaque, saliva, and the vaginal tracts of humans and other mammals.

Lactobacilli are a rather diverse group of bacteria, as is illustrated by their large GC content, which ranges from 32 to 53 mol% (3). To date, greater than 70 different species of Lactobacillus have been identified; of these, only 34 have been identified to the species level (2, 3).

Section snippets

Pathogenesis and Clinical Significance

Lactobacilli are required to maintain a healthy GI tract and are not usually considered to be pathogens in the healthy host except when associated with dental caries (5, 6). They are considered protective organisms and are thought to inhibit the growth of pathogenic organisms via the production of lactic acid and other metabolites. The clinical significance of isolating Lactobacillus from a normally sterile site is the subject of debate. When isolated by the microbiology laboratory, some

Endocarditis

Infective endocarditis (IE) with or without associated bacteremia is the most common infection caused by lactobacilli reported in the literature (1, 6, 12, 13, 14). Many of the patients who develop Lactobacillus IE have recent histories of dental procedures or poor dentition, suggesting that this could be considered a risk factor, especially in those patients with underlying immunosuppression (1, 12, 15). Patients who develop IE also are also likely to have underlying valvular heart disease (7,

Bacteremia

Lactobacillus bacteremia has a wide array of clinical presentations; patients can have severe sepsis or be asymptomatic (10, 5, 17, 19, 20, 31). Fever is the most common symptom in most patients, followed by leukocytosis and rigors (7).Secondary lactobacillemias have been reported in patients who have abscesses, endocarditis, and pneumonia (10, 15, 17, 19, 20, 21, 22).

Lactobacillus bacteremias are believed to occur due to translocation of bacteria across the intestinal mucosa (11, 23).

Localized Infections

Localized infections due to Lactobacillus spp. are also increasingly reported. Abdominal abscesses, pneumonia, other pulmonary infections, and peritonitis are the most commonly described in the literature (1). Once again, underlying immunosuppression is the major risk factor in these infections (5). Other risk factors are diabetes and renal failure (1).

Treatment strategies for localized infections differ from those for IE and bacteremia, with most clinical isolates showing susceptibility to

Probiotics

Due to a resurgence of interest in “all-natural” products to treat illness, Lactobacillus sp. supplemented products have gained popularity. These products are collectively termed probiotics. A probiotic is defined as a dietary supplement consisting of living microorganisms that are found in normal flora and have little, if any, pathogenicity (27, 28). When used, these products are believed to have positive benefits for health by preventing or treating disease. Probiotics have been used in a

Antimicrobial Properties

As typical flora in the GI and urogenital tracts, lactobacilli have been shown to prevent pathogenic bacteria from causing infection. These bacteria exhibit several properties that make them useful for preventing infectious disease. One such property is the production of low-molecular-weight antimicrobials, also known as bacteriocins (41). Bacteriocins are small proteins produced by bacteria that can have toxic effects on other bacteria (42, 43). These proteins are usually active against

Food Industry

Lactobacilli and other lactic acid-producing bacteria are also important organisms used in industrial food production (49). They are used to ferment common foods, such as yogurt, cheese, pickles, sauerkraut, and sourdough bread (49, 50). Fermentation produces lactic acid that causes a drop in pH. This drop in pH then inhibits the growth of putrefactive and pathogenic bacteria. In addition, these organisms also increase the nutritional value of fermented foods. This occurs because lactic

Summary

Lactobacilli are an important part of the human flora, but they can be pathogenic under certain conditions. The most common infections caused by lactobacilli are bacteremias, IE, and abscesses. These infections tend to occur in immunosuppressed patients or those patients with underlying anatomic defects. Given the difficulty in isolating these bacteria and the possibility that they may often be misidentified, with increasing improvements of our laboratory technology it is possible that

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