Aerobic Non-spore-forming Gram-Positive Bacilli

Aerobic Non-spore-forming Gram-Positive Bacilli

I. Listeria monocytogenes

Introduction

• Description: Listeria monocytogenes is a pathogenic bacterium belonging to the genus Listeria.

• Species: There are nineteen recognized species of Listeria.

• Habitat: Listeria is ubiquitous in the environment, found in soil, water, and decaying vegetation.

• Zoonosis: Common reservoirs include cattle, sheep, and goats.

• Epidemiology: Over 1200 cases of listeriosis are reported each year in the United States, leading to more than 200 deaths annually.

Virulence Mechanisms

• Facultative Intracellular Parasite (FIP): Listeria monocytogenes can survive and replicate inside host cells, evading the immune response.

• Listeriolysin O: A hemolysin that damages the phagosome membrane of host cells, aiding the bacterium's escape to the cytoplasm.

Clinical Manifestations

• Healthy Adults: Often experience mild infections or may be asymptomatic.

• Immunocompromised Persons: Can develop gastroenteritis, septicemia, and meningitis.

• Pregnant Women & Neonates: Particularly vulnerable; can lead to severe outcomes for fetuses and newborns.

Diagnosis

• Gram Stain: Used on samples including cerebrospinal fluid (CSF), amniotic fluid, and blood cultures.

• Cultures: Performed on sheep blood agar (SBA) with incubation in CO2.

• Cold Enrichment: Process involving incubation at 4°C to enhance growth of Listeria.

• Colony Morphology: Colonies resemble those of Group B Streptococcus and show beta-hemolysis.

• ### Identification

  • Biochemical Tests:

  • Catalase (+): Important for initial identification.

  • Beta-haemolytic: Observed in culture on SBA.

  • Motility: Visible tumbling motility at room temperature and "umbrella" formation in semi-solid media.

  • Esculin (+): Positive result in specific tests.

  • Molecular Panels: Include multiplex PCR for accurate identification.

II. Corynebacterium

Introduction

• Familial Association: Family Corynebacteriaceae.

• Pathogenic Species: Corynebacterium diphtheriae, which has three biotypes: gravis, intermedius, and mitis.

Pathogenesis

• Toxin Production: Requires a bacteriophage carrying the tox gene, with regulation by iron availability.

• Toxin Activity: Catalyzes ADP ribosylation of diphthamide in elongation factor 2 (EF-2), causing its inactivation.

Clinical Manifestations

• Respiratory Diphtheria: Infection of the upper respiratory tract, affecting the tonsils and pharynx.

• Cutaneous Diphtheria: Infection of the skin, with systemic toxicity being rare (1-2% of cases).

Diagnosis

• Symptoms: Clinical presentations guide preliminary diagnosis.

• Cultures: Gentle swabbing of throat and edges of membranes for culture.

• Toxigenic Studies: Required to demonstrate toxin production in the isolate.

Identification

• Bacterial Isolation:

• Cystine Tellurite Agar: Used with sheep red blood cells, results in gray or black colonies.

• Tinsdale Agar: Specific for corynebacteria.

• Loeffler's Serum Agar: Nonselective medium also used for isolation.

• Gram Stain: Characteristic palisade arrangement viewed under microscope.

Biochemical Characteristics

• Nitrate Positive: Indicates the ability to reduce nitrate.

• Urea Negative: Cannot hydrolyze urea.

• MALDI-TOF: A highly accurate method for identification of corynebacteria.

Toxigenic Studies

• Can be conducted in animal models or with the Elek plate method for assessing toxin production.

Vaccine

• DTaP Vaccine: For children under 7 years, requires 5 doses.

• Tdap Vaccine: For individuals 7 years and older, a single dose recommended every pregnancy or every ten years (booster).

Treatment

• Antitoxin Administration: Given to neutralize diphtheria toxin.

• Antibiotics: Penicillin or erythromycin are standard treatments.

C. jeikeium

• Habitat: Found in healthcare-associated infections (HAIs).

• Pathogenesis: Commonly isolated from blood, wounds, CSF, peritoneal fluid, and urine.

• Identification: Nitrate negative, urease negative, and grows well on SBA.

• Treatment: Often multi-drug resistant; vancomycin or daptomycin are drugs of choice, guided by antimicrobial susceptibility testing (AST).

• ### C. urealyticum

  • Pathogenesis: Associated with urinary tract infections (UTIs).

  • Identification: Urine cultures grow coryneform GPB, nitrate negative, urease positive.

  • Treatment: Susceptibility varies, perform AST, and vancomycin is the drug of choice for UTI.

III. Arcanobacterium

• ### Introduction

  • Species: Five recognized species, with A. haemolyticum being the most clinically significant.

  • Clinical Manifestations: Commonly associated with pharyngitis, particularly in teenagers and young adults.

  • Identification: Characterized by small beta-hemolytic colonies, human isolates are catalase negative, and the Gram stain shows Gram-positive bacilli (GPB).

IV. Rhodococcus

• ### Introduction

  • Classification: Member of the actinomycetes family.

  • Pathogenic Species: Rhodococcus equi, which is primarily a soil inhabitant.

  • Pathogenesis: Typically presents as zoonosis.

  • Identification: Colonies are typically pink/salmon-colored with age and may be mucoid, nitrate positive.

V. Erysipelothrix rhusiopathiae

• ### Pathogenesis

  • Zoonotic Infections: Associated with rare infections known as erysipeloid.

  • Identification: Growth on SBA with CO2 incubation, producing H2S, identified via MALDI-TOF or sequencing.

VI. Nocardia

Introduction

• Family: Nocardiaceae, categorized as aerobic actinomycetes.

• Habitat: Found in soil and on plants.

• Pathogenesis: Infection termed nocardiosis, associated with generally weak virulence; N. nova, N. farcinica, and N. brasiliensis are the most important pathogens.

Clinical Manifestations

• Pulmonary Nocardiosis: The most common form of the disease.

• Cutaneous Infections: Occur from direct skin inoculation, potentially leading to mixed mycetoma when combined with Scedosporium boydii.

• Severe Forms: Refers to a condition known as madura foot (actinomycotic mycetoma).

• ### Diagnosis

  • Gram Stain: Used in identification processes.

  • Cultures:

  • Pulmonary Form: Samples often taken from sputum, bronchoalveolar lavage (BAL), or biopsy/wound exudate for mycetoma.

  • Identification: Uses modified (partially) Acid-fast stain which is positive, along with biochemical testing and MALDI-TOF sequencing for confirmed diagnostics.

VII. Bacillus

Introduction

• Characteristics: Belongs to the family Bacilleaceae; these are spore-formers, catalase positive, either obligate aerobes or facultative anaerobes, and generally motile.

• Epidemiology: Soil inhabitants, typically exhibit low virulence.

B. anthracis

• Virulence Mechanisms:

• Capsule: Composed of D-glutamic acid.

• Exotoxins (Anthrax Toxin): Comprises three components:

  • EF (Edema Factor): Functions as an adenylate cyclase.

  • LF (Lethal Factor): Acts as a protease.

  • PA (Protective Antigen): Binds to cellular receptors.

• Facultative Intracellular Parasite: Capable of surviving within phagocytes.

• Spore Formation: Allows resistance to harsh environmental conditions, germinating when favorable conditions return.

• Clinical Manifestations:

• Zoonotic Disease: Commonly noted.

• Cutaneous Anthrax: Initiated with a papule evolving into an eschar; serious systemic forms can occur leading to eschar.

• Pulmonary Anthrax (Woolsorter's Disease): Result of inhalation of spores, characterized by widened mediastinum on imaging.

• Gastrointestinal Anthrax: Occurs upon ingestion of spores.

• Bioterrorism:

• Use of B. anthracis: Noted as an advantageous agent for bioterrorism.

• Terrorist Attacks (2001): Documented 11 cases each of cutaneous and pulmonary anthrax among postal workers.

• Diagnosis and Characteristics:

• Culture Considerations: Extreme caution during handling due to risks involved; requires working in a biosafety cabinet (BSC).

• Tier 1 Select Agent: Recognized as a pathogen requiring specific regulatory measures.

• Laboratory Accidents (LAIs): Notably risky for personnel.

• Clinical & Environmental Samples: Only to be analyzed by FBI and DOD, with onsite polymerase chain reaction (PCR) techniques.

• Preliminary Testing: Conducted at sentinel (clinical) laboratories assessing colony morphology; notable for being non-hemolytic and appearing as "medusa head" or tenacious colonies.

• Gram Stain: Utilized for identification, revealing non-motile attribute.

• Confirmatory Identification: At reference (public health) laboratories utilizing methods such as fluorescent antibody staining, bacteriophage (gamma phage) lysis, and PCR.

• Treatment: Inclusion of antibiotics such as ciprofloxacin or doxycycline, in addition to other drugs.

• Vaccine:

• Animal Vaccine: Developed for B. anthracis.

• Human Vaccine (BioThrax): Controversial, primarily effective against pulmonary anthrax, yet concerns regarding safety persist.

• Target Population: Includes military personnel, researchers, and others at risk of exposure.

B. cereus

• Clinical Manifestations:

• Food Poisoning:

  • Emetic Form: Caused by cereulide, a stable toxin.

  • Diarrheal Form: Results from enterotoxins which are labile.

• Systemic Infections and Wounds: Can lead to eye infections, pneumonia, and various wound infections.

• Diagnosis and Characteristics:

• Motility: Characterizes the species.

• Beta-hemolytic: Observed in culture.

• Penicillin Resistance: Notably resistant to the antibiotic penicillin.

Other Bacillus spp.

• Introduction: Increasing recognition of opportunistic infections associated with Bacillus species.

• Species Encountered: Includes B. subtilis, B. thuringiensis, and others, excluding B. anthracis.

• Criteria for Clinical Significance:

• Isolation from compromised patients.

• Colonies originating exclusively from the original streak line(s).

• Isolation from normally sterile sites.

• Observation of gram-positive bacilli on direct smear.

• Repeated isolation from patients and environmental sources.

• Demonstrating isolation in multiple patients.