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Introduction

Lactococcus is a genus of Gram-positive, catalase-negative, non-spore-forming cocci, usually arranged in pairs or short chains. They are facultative anaerobes and are best known for their beneficial role in dairy fermentation (e.g., cheese and buttermilk production). Although generally regarded as low-virulence organisms, Lactococcus species can act as opportunistic pathogens, especially in immunocompromised individuals.

Pathogenicity

• Normally part of food-associated and environmental microbiota

• Opportunistic infections reported in:

  1. Immunocompromised patients

  2. Elderly individuals

  3. Patients with valvular heart disease, malignancy, or indwelling devices

• Reported clinical conditions:

  1. Bacteremia

  2. Infective endocarditis

  3. Urinary tract infections

  4. Wound and soft tissue infections

• Virulence is low, and disease often results from host factors rather than aggressive bacterial toxins

Laboratory Diagnosis

Specimen

• Blood, urine, wound swab, body fluids (depending on infection site)

Microscopy

1. Gram stain: Gram-positive cocci in pairs or short chains

2. Catalase test: Negative (helps differentiate from Staphylococcus)

Culture Characteristics

1. Growth on Blood agar: small, grayish, non-hemolytic or α-hemolytic colonies

2. Growth on SDA is possible (acid-tolerant), sometimes mistaken for yeasts/bacteria mix

3. Does not grow well in high salt (unlike Enterococcus)

Biochemical Tests

1. Catalase negative

2. PYR: usually negative

3. Bile esculin: negative

4. Sugar fermentation tests may assist

Advanced Identification

1. MALDI-TOF MS (preferred)

2. 16S rRNA sequencing (confirmatory)

Treatment

• Most isolates are susceptible to beta-lactam antibiotics

• Commonly effective drugs:

  1. Penicillin

  2. Ampicillin

  3. Amoxicillin

  4. Ceftriaxone

• Severe infections (e.g., endocarditis):

  1. Beta-lactam ± gentamicin (based on susceptibility)

• Always perform antimicrobial susceptibility testing (AST) due to variable resistance

Prevention

1. Good food hygiene and handling practices

2. Proper care of indwelling catheters and medical devices

3. Early identification and treatment in high-risk patients

4. Strict aseptic techniques in clinical settings

Keynotes

1. Gram-positive, catalase-negative cocci

2. Commonly misidentified as Streptococcus or Enterococcus

3. Opportunistic pathogen with low intrinsic virulence

4. Increasingly detected due to MALDI-TOF and molecular methods

5. Clinical relevance depends on the host’s immune status

Further Readings

• https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/lactococcus
• https://lpsn.dsmz.de/genus/lactococcus
• https://link.springer.com/chapter/10.1007/978-1-4615-5817-0_6
• https://www.ncbi.nlm.nih.gov/datasets/genome/GCA_023809745.1
• https://en.wikipedia.org/wiki/Lactococcus
• https://wid.wisc.edu/l-lactis-the-unsung-hero-of-wisconsins-world-famous-cheese/
• https://www.researchgate.net/publication/374334322_Lactococcus_lactis_a_bacterium_with_probiotic_functions_and_pathogenicity
• https://pmc.ncbi.nlm.nih.gov/articles/PMC12298479/