Nonsporing anaerobes – Definition, Classification, Examples, Diagnosis

What are Nonsporing anaerobes?

• Nonsporing anaerobes are a group of bacteria that thrive in environments with little to no oxygen. Unlike spore-forming bacteria, these organisms do not produce spores, making them distinct in both their survival mechanisms and their response to environmental stressors.
• These bacteria are often part of the normal flora in humans and animals, particularly in areas with limited exposure to oxygen. Common habitats include the gastrointestinal tract, genital tract, oropharynx, and the skin. In fact, the abundance of nonsporing anaerobes in the gut is notable, as they can outnumber aerobic bacteria like Escherichia coli by a ratio of 1,000:1.
• Under normal circumstances, these bacteria do not cause harm. However, when the body’s immune system is compromised, or when tissues become damaged or necrotic, these bacteria can act as opportunistic pathogens. In such cases, they can contribute to infections, often as part of polymicrobial mixtures. These infections can include intra-abdominal sepsis, pelvic inflammatory disease, chronic sinusitis, and abscesses in organs like the liver and brain. Nonsporing anaerobes such as Bacteroides species, particularly Bacteroides fragilis, are commonly implicated in abdominal and gynecological infections, while Fusobacterium species can be linked to conditions like dental abscesses and Vincent’s angina.
• Isolation of nonsporing anaerobes for diagnostic purposes can be challenging due to their sensitivity to oxygen. Therefore, specimen collection must be performed with caution to avoid contamination by normal flora. These bacteria require specialized anaerobic culture conditions and transport media to thrive in the laboratory. Advances in molecular diagnostics, including nucleic acid amplification tests (NAATs), have made it easier to detect these organisms rapidly, bypassing some of the traditional limitations in culture-based methods.
• The treatment of infections caused by nonsporing anaerobes typically involves the use of antibiotics. Commonly prescribed antibiotics include metronidazole, penicillin, and chloramphenicol. The choice of antibiotic depends on the specific species involved in the infection.
• Nonsporing anaerobes are of significant clinical importance, particularly in the context of infections resulting from necrotic tissue. Although these organisms are part of the normal microbiota in several body regions, their potential to cause severe infections when the host’s defenses are weakened makes them an important group of pathogens to monitor and manage in clinical practice.

Classification of Non-Sporing Anaerobes

Non-sporing anaerobes are microorganisms that thrive without the need for oxygen and do not produce spores. Based on their morphology and staining properties, these bacteria are classified into several categories. Here’s a breakdown of how they are classified according to their Gram reaction and structure:

Gram-Negative Bacilli:

• Bacteroides: Commonly found in the mouth and intestines, with B. fragilis being one of the most significant species.
• Prevotella: Often part of the normal flora in the mouth and respiratory tract.
• Porphyromonas: Typically found in the mouth and can be involved in periodontal diseases.
• Fusobacterium: Includes species like F. necrophorum, known for its role in various infections.
• Leptotrichia: Found in the mouth and respiratory tract.
• Peptococci: These Gram-negative cocci are part of the normal flora and can cause infections in certain conditions.

Gram-Positive Bacilli:

• Eubacterium: Often part of the normal flora in the mouth and intestines, playing a role in breaking down complex molecules.
• Propionibacterium: Known for being part of the skin’s microbiome, with P. acne being a common species.
• Lactobacillus: These bacilli are essential in the human body, especially in the intestines and vagina, for maintaining an acidic environment.
• Mobiluncus: Found in the genital tract, associated with bacterial vaginosis.
• Bifidobacterium: A significant part of the normal gut flora, particularly in infants, aiding in digestion.
• Actinomyces: These bacteria are involved in chronic infections like actinomycosis.

Cocci:

• Peptococci: These Gram-positive cocci are often found in infections like abscesses and wound infections.
• Peptostreptococci: Frequently isolated from pleuropneumonia and brain abscesses, these cocci are common in human infections.
• Veillonella: These Gram-negative cocci can occasionally cause infections, though they are generally non-pathogenic.

Spirochetes:

• Treponema: These bacteria, which include T. pallidum, the cause of syphilis, are non-sporing and anaerobic.
• Borrelia: Known for causing Lyme disease and relapsing fever, these spirochetes are also non-sporing anaerobes.

Non-Sporing Anaerobes in Soil and Water:

Certain non-sporing anaerobes can also be found outside the human body, such as:

• Butyri vibrios
• Methanobacteria

These bacteria are generally found in environmental sources like soil and water, contributing to processes like methane production.

Classification Based on Morphology and Habitat

1. Cocci (Anaerobic):
   • Peptococcus: Found in pairs or clusters, they cause infections like wound sepsis and urinary tract infections.
   • Peptostreptococcus: Arranged in chains, they are commonly associated with pleuropneumonia and brain abscesses.
   • Sarcina: These large cocci form packets and are usually harmless, found in soil and vegetarian feces.
   • Veillonella: These small, Gram-negative cocci can be part of the normal flora but may invade the bloodstream post-surgery.
2. Bacilli:
   • Anaerobic Gram-Negative Bacilli:
     • Bacteroides: They dominate the intestinal and oral microbiota, with B. fragilis being a key species.
     • Fusobacterium: Found in the mouth and respiratory tract, it plays a role in infections like Lemierre’s syndrome.
     • Leptotrichia: Present in the oral cavity and respiratory tract.
   • Anaerobic Gram-Positive Bacilli:
     • Lactobacillus: Present in the mouth, gut, and vagina, these bacteria help ferment food products and maintain an acidic pH in the vagina.
     • Bifidobacterium: A Y-shaped bacillus, they are part of the normal gut flora and rarely pathogenic.
     • Propionibacterium: Commonly found on the skin, particularly in acne pustules, though their pathogenicity is still under research.

Anaerobic Cocci

Anaerobic cocci are bacteria that thrive in environments devoid of oxygen. There are two main categories to consider: Gram-positive cocci and Gram-negative cocci. These organisms are part of the normal flora in various body regions but can cause infections when they spread to normally sterile sites.

Anaerobic Gram-Positive Cocci

• Peptostreptococcus is the genus most associated with anaerobic Gram-positive cocci.
• These cocci are small, measuring between 0.2 to 2.5 micrometers.
• They are aerotolerant, meaning they can survive in the presence of oxygen, particularly when the environment contains about 10% CO2. This makes them capable of growing well even in slightly oxygenated environments.
• These bacteria naturally colonize the oral cavity, gastrointestinal tract, genitourinary tract, and skin.
• When they move from their typical sites to other normally sterile areas, they can lead to a range of infections:
  • Cellulitis and soft tissue infections from skin colonization.
  • Intra-abdominal infections from the intestinal flora.
  • Pelvic abscesses, salpingitis, and endometritis due to spread from the genital tract.
  • Bone infections or visceral organ infections when the bacteria enter the bloodstream.
• Two species are notably associated with specific infections:
  • Peptostreptococcus anaerobius: Linked with puerperal sepsis.
  • Peptostreptococcus magnus: Commonly found in abscess infections.
• Laboratory Diagnosis:
  • The diagnosis is often based on the isolation of the bacteria from clinical specimens through culture.
  • Specimens are typically grown on blood agar or other enriched media. These cultures require a prolonged incubation period of 5–7 days in anaerobic conditions, as opposed to other anaerobic bacteria that grow in 1–2 days.
  • When collecting specimens, it’s essential to avoid contamination by the normal flora of the mucosal membrane and skin, where Peptostreptococcus is typically found.
  • In pus samples from suppurative lesions, these cocci often appear in large numbers, making Gram staining of the pus useful for diagnosis.
• Antibiotic Sensitivity:
  • Generally sensitive to imipenem, chloramphenicol, and metronidazole.
  • Intermediate sensitivity is noted for broad-spectrum cephalosporins, tetracyclines, and clindamycin.
  • Resistant to streptomycin and gentamicin.
  • Mixed infections are common, with these cocci often found alongside anaerobic Gram-negative bacilli and clostridia, so treatment usually involves broad-spectrum antibiotics that can target all these pathogens.

Anaerobic Gram-Negative Cocci

• The genus Veillonella includes anaerobic Gram-negative cocci.
• These cocci are typically found in pairs, short chains, or clusters.
• They naturally inhabit the mouth, intestines, and genitourinary tract.
• Veillonella parvula is the most commonly identified species in clinical samples.
• While Veillonella parvula is frequently recovered from clinical specimens, its precise role in the pathogenesis of disease is still not well understood.

Anaerobic Bacilli

Anaerobic Gram-Positive Bacilli

Anaerobic Gram-positive bacilli are a diverse group of bacteria that thrive without oxygen. While many species of this group are part of the natural flora on the skin and mucosal membranes, a few are associated with clinical infections.

The primary genera of interest in clinical settings include Propionibacterium, Lactobacillus, Mobiluncus, Actinomyces, Eubacterium, and Bifidobacterium. Among them, Propionibacterium, Lactobacillus, and Mobiluncus are the most clinically significant.

Propionibacterium

• Propionibacterium species are typically anaerobic or aerotolerant.
• These bacteria are found in areas like the skin, oropharynx, conjunctiva, external ear, and female genital tract.
• Gram-positive bacilli appear in short chains or clumps, are nonmotile, and catalase-positive.
• They ferment carbohydrates and produce propionic acid, which is why they are named Propionibacterium.
• The most notable species is Propionibacterium acnes, which is the leading cause of acne in teenagers and young adults.
• It also causes infections in the heart valves, joints, cerebrospinal fluid shunts, and can lead to cellulitis.
• Propionibacterium propionicus is another important species but less frequently involved in clinical infections.
• Laboratory growth: Propionibacterium are non-fastidious and can grow on simple media, though they require 3–5 days to form colonies.
• Clinical diagnosis: When isolated from clinical specimens, Propionibacterium should be interpreted cautiously, especially in the case of acne or as a contaminant in blood or cerebrospinal fluid cultures.
• Treatment: Acne treatment is most effective with benzoyl peroxide and antibiotics. Erythromycin and clindamycin are effective against Propionibacterium.

Lactobacillus

• Lactobacillus species are facultatively anaerobic, meaning they can grow with or without oxygen.
• These bacteria are a common part of the normal flora in the mouth, stomach, intestines, and vagina.
• While they are often isolated from blood and urine cultures, the presence of Lactobacillus in urine usually points to contamination from the urethra.
• Infections: Lactobacillus can occasionally cause endocarditis, opportunistic septicemia, and bacteremia in immunocompromised patients or after genitourinary procedures.
• They are generally resistant to vancomycin but susceptible to a combination of penicillin and aminoglycosides.

Mobiluncus

• Mobiluncus species are obligate anaerobes and are classified as Gram-positive despite their Gram-variable staining.
• These curved bacilli with tapering ends are often found in the genitourinary tract.
• In women suffering from bacterial vaginitis, Mobiluncus can be found in large numbers, though its exact role in causing the condition remains unclear.
• These bacteria require enrichment media like blood agar with rabbit or horse blood for growth.
• Gram-staining patterns are typically diagnostic, and Mobiluncus is susceptible to erythromycin, ampicillin, clindamycin, and vancomycin, but resistant to colistin.

Bifidobacterium and Eubacterium

• Bifidobacterium and Eubacterium are often isolated from the oropharynx, large intestine, and vagina.
• They are typically considered contaminants in clinical specimens but can sometimes be associated with infections.
• Repeated isolation of these bacteria without other pathogens may indicate their potential role in causing disease.

Anaerobic Gram-Negative Bacilli

Anaerobic Gram-negative bacilli play a significant role in a wide range of infections. These bacteria are part of the Bacteroidaceae family, which includes the genera Bacteroides, Fusobacterium, Porphyromonas, and Prevotella.

• Bacteroides are the most prominent in clinical infections, especially Bacteroides fragilis, which is a major cause of intra-abdominal infections. Other species like Bacteroides distasonis, Bacteroides vulgatus, and Bacteroides thetaiotaomicron are more commonly found in the gastrointestinal tract but don’t typically cause infections.
• Fusobacterium species are another important group. These are thin and long, and while they can be difficult to culture due to their slow growth, they contribute significantly to infections in the sinuses and ears.
• Prevotella and Porphyromonas are also part of this group. Prevotella species, although small and elongated, can cause various chronic infections, particularly in the sinuses and periodontal tissues.

Key Characteristics of Anaerobic Gram-Negative Bacilli

• These bacteria are characterized by their Gram-negative cell walls and lipopolysaccharide (LPS) as the major component of the surface.
• Bacteroides fragilis is pleomorphic in size and shape, with a typical Gram-negative wall structure. However, unlike Fusobacterium, the LPS of B. fragilis doesn’t exhibit endotoxin activity.
• Polysaccharide capsules surround the cell wall of Bacteroides fragilis, helping the bacteria evade phagocytosis by immune cells and facilitating adherence to surfaces in the body.
• Anaerobic Gram-negative bacilli tend to be part of the normal flora in various body sites like the skin, mouth, intestines, and vagina, but can cause infections if they move from their typical habitats.

Pathogenicity

• The ability to inactivate immunoglobulins, cause tissue destruction, and resist oxygen toxicity is vital in the pathogenesis of these infections.
• Bacteroides fragilis produces a variety of virulence factors that contribute to its pathogenicity, including the ability to resist immune system attacks and promote tissue damage.
• Bacteroides fragilis causes more than 80% of intra-abdominal infections and is often responsible for pelvic inflammatory disease, endometritis, and abscesses in the female genital tract.

Clinical Manifestations

• Infections caused by anaerobic Gram-negative bacilli typically involve a polymicrobial mixture, meaning that both anaerobic and aerobic bacteria are often present.
• These infections are characterized by foul-smelling pus, pronounced cellulitis, and sometimes fever and toxemia.

Diagnosis

• Diagnosing infections caused by these bacteria requires careful interpretation of culture results, as these organisms are commonly part of normal flora.
• Specimens for culture must be collected from non-normal sites, kept moist, and transported in an oxygen-free environment.
• Blood agar with yeast extract, hemin, vitamin K, and neomycin is commonly used for culture, and incubation is done under anaerobic conditions.
• Gas-liquid chromatography can be used to identify metabolic products of anaerobes, supporting diagnosis.

Treatment

• Surgical intervention is often required alongside antibiotic therapy.
• Metronidazole is the antibiotic of choice for infections caused by these bacteria, particularly Bacteroides species.
• Some species like Bacteroides fragilis, Porphyromonas, and Prevotella are resistant to penicillins and cephalosporins due to the production of beta-lactamase. This resistance can be managed by using beta-lactamase inhibitors or higher concentrations of carbenicillin, piperacillin, or imipenem.
• Resistance to clindamycin can also be a concern due to plasmid-mediated resistance in some Bacteroides species.

Infections caused by anaerobic Gram-negative bacilli are serious, but with proper diagnosis and treatment, including both antibiotics and surgical intervention, they can be managed effectively.

Clinical Syndromes Due to Non-Sporing Anaerobes

Non-sporing anaerobes are involved in a variety of infections, often in polymicrobial environments. These infections can arise from different body sites, typically when bacteria from their usual habitat spread to normally sterile areas. Non-sporing anaerobes contribute to conditions ranging from abscesses to chronic infections. Here’s a breakdown of the major clinical syndromes associated with these bacteria.

• Intra-Abdominal Sepsis
  • Caused by spontaneous bowel perforations or surgical leakage, this condition often leads to abscess formation in the abdomen or liver.
  • Non-sporing anaerobes are commonly involved in these abscesses, often complicating post-surgical recovery or spontaneous perforations in the gastrointestinal tract.
• Pelvic Inflammatory Disease (PID)
  • Non-sporing anaerobes play a significant role in PID, which can result from septic abortion, prolonged rupture of membranes, complicated cesarean sections, or retained products of conception.
  • These infections can lead to severe complications if not managed properly, including infertility or chronic pelvic pain.
• Non-Specific Vaginosis Syndrome
  • Imbalance in vaginal flora, often involving non-sporing anaerobes, can lead to bacterial vaginosis.
  • This condition is marked by a shift from healthy lactobacilli to a mix of anaerobic organisms, often causing discomfort and increasing the risk of other infections.
• Liver Abscesses and Biliary Sepsis
  • Non-sporing anaerobes are involved in polymicrobial liver abscesses and biliary tract infections.
  • These infections are often seen in individuals with underlying liver disease or following abdominal procedures.
• Pneumonia and Lung Abscesses
  • Aspiration pneumonia, often linked to carcinomas or foreign-body obstruction, has a notable anaerobic component.
  • Over time, this can lead to the formation of lung abscesses, a serious condition that requires prompt treatment.
• Brain Abscesses
  • Anaerobic bacteria, including non-sporing species, are frequently involved in the formation of brain abscesses.
  • These infections are typically the result of direct spread from other infected areas, such as the sinuses or the mouth.
• Chronic Paranasal Suppuration
  • Conditions like chronic otitis media and chronic sinusitis often harbor non-sporing anaerobes, contributing to persistent infections.
  • These infections can be difficult to treat and may require long-term management strategies.
• Chronic Skin Ulcers
  • Non-sporing anaerobes can colonize chronic skin ulcers, contributing to slow-healing wounds and complicating recovery.
  • Proper wound care and appropriate antibiotic therapy are crucial in managing these infections.
• Tropical Ulcers
  • Fusobacterium ulcerans, a non-sporing anaerobe, is a rare but known cause of tropical ulcers.
  • These ulcers are typically found in regions where conditions support the growth of anaerobic bacteria, often requiring specialized care.

Pathogenesis of Non-Sporing Anaerobes

Non-sporing anaerobes are typically part of the normal flora in the human body but can cause infections when they enter sterile sites. These infections often begin when anaerobes migrate from their usual habitat to areas where they shouldn’t be. Here’s how the process unfolds.

• Endogenous Infection
  • Non-sporing anaerobes usually come from the body’s own flora.
  • These bacteria can escape into sterile areas after events like hollow viscus perforation (e.g., perforation of the large intestine), allowing them to infect otherwise healthy tissues.
• Anaerobic Conditions Facilitate Growth
  • Ischemia (such as from a strangulated hernia) is a key factor that allows anaerobic bacteria to thrive.
  • When tissue blood flow is restricted, it creates the perfect environment for anaerobes to grow, as they thrive in oxygen-deprived conditions.
  • Facultative bacteria can also help set the stage for anaerobic infections by consuming available oxygen and creating anaerobic conditions, as seen in deep skin ulcers or intraperitoneal infections.
• Toxic Products and Enzymatic Damage
  • Once non-sporing anaerobes begin to grow, they release toxic metabolic products and proteolytic enzymes.
  • These products not only cause direct damage to surrounding tissues but also fuel the spread of infection, promoting further bacterial multiplication.
• Vicious Cycle of Inflammation and Tissue Damage
  • Inflammatory cells in the infected area release reactive oxygen intermediates as part of the immune response.
  • However, these intermediates can actually worsen tissue damage, creating a vicious cycle.
  • The damage accelerates anaerobic sepsis, which becomes rapidly progressive if not controlled.

Laboratory Diagnosis of Non-Sporing Anaerobes

Diagnosing non-sporing anaerobes can be tricky. These bacteria are picky eaters, sensitive to oxygen, and often grow slowly. But the right approach in the lab can yield results.

• Specimen Collection
  • Pus is the preferred specimen. It should be collected in the theatre or at the bedside to minimize exposure to oxygen.
  • Swabs aren’t ideal because they dry out quickly, which can lead to a loss of viable bacteria.
  • After collection, specimens must be transported quickly to the lab using an anaerobic transport system to prevent the oxygen-sensitive bacteria from dying off before analysis.
• Growth and Culturing
  • Non-sporing anaerobes require anaerobic conditions for growth, which means specialized techniques are needed to isolate them in the lab.
  • These bacteria are nutritionally fastidious, meaning they need specific conditions and nutrients to thrive.
  • Culturing them can be slow, so patience is key during the process.
• Identification Methods
  • Traditional phenotypic tests are used to identify anaerobic species. This involves looking at their morphology, biochemical properties, and growth patterns.
  • One of the most reliable ways to study these bacteria is by analyzing their metabolic end-products through gas-liquid chromatography. This can reveal key metabolic pathways and byproducts unique to specific species.
  • Molecular techniques are increasingly used. The most promising tool is the amplification of the 16S rRNA gene using nucleic acid amplification tests (NAATs). This method can quickly and accurately identify anaerobic bacteria based on their genetic material.

Management of Non-Sporing Anaerobes

When dealing with infections caused by non-sporing anaerobes, proper management is key. Treatment often involves a combination of surgical intervention and antimicrobial therapy.

• Surgical Intervention
  • Abscess drainage: When an abscess forms due to anaerobic infection, draining the pus is crucial to reduce infection and prevent further complications.
  • Closure of perforations: In cases where there’s a perforation in a hollow organ (like the intestine), it’s essential to close the perforation to prevent the spread of bacteria to sterile body areas.
  • Resection of gangrenous tissue: If tissues become necrotic or gangrenous, they must be removed to halt the infection’s spread.
  • Debridement of non-viable tissue: Ulcers or wounds with non-viable tissue need to be cleaned out to ensure the infection doesn’t persist.
  • Treatment of coexisting infections: If there are other infections present, they need to be addressed alongside the anaerobic infection.
• Antimicrobial Therapy
  • Metronidazole is the go-to drug for treating anaerobic infections. It’s highly effective against a wide range of anaerobic bacteria, making it a first-line choice in many cases.