Answer: Endocarditis due to Lactococcus lactis subsp. lactis. Because the sensitivity to cephalosporins did not fit the identification made, further characterization was done using mass spectrometry with the Vitek MS system (bioMérieux), which identified the isolated microorganism as Lactococcus lactis subsp. lactis with a 99.9% confidence value. The identification was corroborated at the Microbiology National Center using DNA sequence analysis of the 16S rRNA gene. Antibiotic sensitivity was tested using the Vitek 2 system (bioMérieux), and the MICs were interpreted using the EUCAST breakpoints for Streptococcus viridans. The isolated microorganism was sensitive to ampicillin, cefuroxime, clindamycin, erythromycin, levofloxacin, teicoplanin, and vancomycin and resistant to trimethoprim-sulfamethoxazole. These findings were confirmed in a new set of blood cultures performed 24 h after the first series. A transesophageal echocardiography was performed, and a vegetation of 6 mm in diameter on the aortic valve was observed. The patient was diagnosed with late endocarditis on a biological aortic prosthesis due to Lactococcus lactis subsp. lactis. The intravenous antibiotic treatment was changed to ampicillin, gentamicin, and vancomycin. Given the persistent low-grade fever in the absence of growth on follow-up blood cultures, a new transesophageal echocardiography was performed. The size of the endocarditis vegetation had increased to 15 mm, and the existence of a possible annular abscess was observed. Intravenous rifampin was added to the antibiotic treatment, and an aortic valve replacement was performed. After surgery, a transesophageal echocardiography was carried out to ensure the normal functioning of the aortic valve prosthesis and the absence of vegetation. The patient was then discharged.
Lactococcus lactis is a fermenting and lactic acid-producing microorganism that is present in raw milk and is widely used in the dairy industry for the manufacture of fermented products, particularly cheese. Its isolation is uncommon in clinical samples, and it is considered a low-pathogenicity opportunistic microorganism. Cases of endocarditis caused by Lactococcus lactis are unusual, and only eight cases have been reported in the literature, two due to Streptococcus lactis (formerly Lactococcus lactis) (1, 2), four due to Lactococcus lactis subsp. cremoris (3–6), and only two due to Lactococcus lactis subsp. lactis (7, 8). Five of the eight patients (62.5%) had no history of heart disease, and the other three patients (37.5%) had prior valve pathologies. One patient died of multiple secondary intracerebral hemorrhages, and four other patients (57.14%) required surgery for pathology resolution.
It is possible that Lactococcus lactis endocarditis had been underdiagnosed, due to the similarity of L. lactis colonies to enterococci in standard culture media and their similar biochemical profiles. Common phenotypic tests used to identify Enterococcus spp., such as growth in 6.5% salt broth, bile esculin hydrolysis, and testing for pyrrolidonyl arylamidase (PYR), give variable results for Lactococcus spp. Both species produce acid from mannitol, but only Enterococcus spp. produce acid from arabinose, raffinose, or sorbitol. The use of mass spectrometry for microbiological diagnosis may contribute to the revision of the pathogenicity data for this microorganism. As far as we know, however, this is the first reported case of infectious endocarditis by Lactococcus lactis subsp. lactis in a patient with a mitral biological aortic prosthesis.
See https://doi.org/10.1128/JCM.03357-15 in this issue for photo quiz case presentation.
REFERENCES
- 1.Wood HF, Jacobs K, McCarty M. 1955. Streptococcus lactis isolated from a patient with subacute bacterial endocarditis. Am J Med 18:345–347. doi: 10.1016/0002-9343(55)90247-3. [DOI] [PubMed] [Google Scholar]
- 2.Mannion PT, Rothburn MM. 1990. Diagnosis of bacterial endocarditis caused by Streptococcus lactis and assisted by immunoblotting of serum antibodies. J Infect 21:317–318. doi: 10.1016/0163-4453(90)94149-T. [DOI] [PubMed] [Google Scholar]
- 3.Pellizzer G, Benedetti P, Biavasco F, Manfrin V, Franzetti M, Scagnelli M, Scarparo C, De Lalla F. 1996. Bacterial endocarditis due to Lactococcus lactis subsp. cremosis: case report. Clin Microbiol Infect 2:230–232. doi: 10.1016/S1198-743X(14)65148-X. [DOI] [PubMed] [Google Scholar]
- 4.Halldórsdóttir HD, Haraldsdóttir V, Bödvarsson A, Thorgeirsson G, Kristjánsson M. 2002. Endocarditis caused by Lactococcus cremosis. Scand J Infect Dis 34:205–206. doi: 10.1080/00365540110080377. [DOI] [PubMed] [Google Scholar]
- 5.Resch M, Schichtl T, Endemann DH, Griese DP, Kasprzak P, Djavidani B, Fleck M, Luchner A, Riegger GAJ. 2008. General aneurysmatosis due to cheese consumption: complications of an endocarditis caused by Lactococcus cremoris. Int J Cardiol 126:e8–e9. doi: 10.1016/j.ijcard.2006.12.068. [DOI] [PubMed] [Google Scholar]
- 6.Lin KH, Sy CL, Chen CS, Lee CH, Lin YT, Li JY. 2010. Infective endocarditis complicated by intracerebral hemorrhage due to Lactococcus lactis subsp. cremoris. Infection 38:147–149. doi: 10.1007/s15010-010-9219-3. [DOI] [PubMed] [Google Scholar]
- 7.Zechini B, Cipriani P, Papadopoulou S, Di Nucci G, Petrucca A, Teggi A. 2006. Endocarditis caused by Lactococcus lactis subsp. lactis in a patient with atrial myxoma: a case report. Diagn Microbiol Infect Dis 56:325–328. doi: 10.1016/j.diagmicrobio.2006.04.011. [DOI] [PubMed] [Google Scholar]
- 8.Rostagno C, Pecile P, Stefàno PL. 2013. Early Lactococcus lactis endocarditis after mitral valve repair: a case report and literature review. Infection 41:897–899. doi: 10.1007/s15010-012-0377-8. [DOI] [PubMed] [Google Scholar]