Thymidine‐auxotrophic Staphylococcus aureus small‐colony variant bacteremia in a patient with cystic fibrosis

Abstract

BackgroundSmall‐colony variants (SCVs) are a morphologic subtype of Staphylococcus aureus that may occur through several mechanisms including auxotrophism for thymidine, hemin, or menadione. Auxotrophic SCV for thymidine fail to synthesize DNA specifically because of mutations in the thymidylate synthase gene. We isolated S. aureus thymidine‐dependent SCVs (TD‐SCV) from blood and respiratory samples of a pediatric patient with cystic fibrosis and pulmonary exacerbation.MethodsNutritional dependence of SCVs on hemin, menadione, and thymidine was evaluated. Antimicrobial susceptibility testing was performed through broth microdilution. Polymerase chain reaction was carried out for mecA, ermA, ermB, ermC, msrA, and msrB resistance genes. DNA sequencing was used to determine mutations in thyA and the multilocus sequence typing to identify genetic relatedness.ResultsMethicillin‐sensitive S. aureus with normal and TD‐SCV phenotypes were isolated from respiratory samples and a TD‐SCV phenotype was isolated from blood culture. Macrolides resistance was attributed to ermC and msrB genes. All isolates belonged to ST398. The thyA gene in S. aureus is 957 nucleotides in length and encodes a protein of 318 amino acids. The TD‐SCV isolates carried a −2 nt frameshift mutation (delta 667GC668) in thyA, creating a stop codon at residue 222 close to the predicted binding site for deoxyuridine monophosphate.ConclusionsThe pathogenesis of SCVs is complex and not fully elucidated. Factors inherent to the patient such as physiological conditions, recurrent infections, or coinfection should be considered. Although SCVs are considered less virulent, they showed the ability to invade and cause bacteremia in the patient.

Emergence of Thymidine-Dependent Staphylococcus aureus Small-Colony Variants in Cystic Fibrosis Patients in Southern Brazil

“…The ST5 and ST398 clones were found more than once. These clones have been previously detected in the context of methicillin-susceptible S. aureus (MSSA) and MRSA isolates with the SCV phenotype from respiratory samples of CF patients ( 31 ); the ST5 clone was described as a community-acquired isolate ( 32 ), and ST398 has been reported in animal and human infections ( 8 ) and may present high virulence ( 33 ).…”

“…Thymidine auxotrophism has emerged as a consequence of genetic mutations in the thyA gene due to prolonged treatment with trimethoprim-sulfamethoxazole ( 7 , 8 ). This antibiotic inhibits the synthesis of tetrahydrofolic acid, which serves as a cofactor for thymidylate synthase (encoded by thyA ), an enzyme required for the conversion of uracil into thymidine ( 3 ).…”

“… S. aureus TD-SCVs have been associated with different chronic infections such as endocarditis, arthritis, osteomyelitis, and rhinosinusitis ( 3 ). Remarkably, despite their low virulence, they were previously isolated in the context of bacteremia ( 8 ).…”

Staphylococcus aureus and Cystic Fibrosis—A Close Relationship. What Can We Learn from Sequencing Studies?

“…The overall trend of the adaptation process seems to be directed towards a persistent and colonizing rather than infecting lifestyle and towards the establishment of a heterogeneous population, which fit into different niches in CF airways and are comparable with the "morphotypes" of P. aeruginosa, which describe different phenotypes [25][26][27]. This is also reflected by the fact that, even in case of an exacerbation, the presence of S. aureus is restricted to the airways and systemic disease is rarely observed [28,29]. The preponderance of colonizing clones results from the suppression of virulence genes for the sake of an intensified expression of adhesins and biofilm formation, granting protection against antibiotic therapy and immune-response [15,30].…”

Unusual and Unconsidered Mechanisms of Bacterial Resilience and Resistance to Quinolones

“…SCVs are naturally occurring variants described in both Gram-negative and Gram-positive bacteria, such as Burkholderia pseudomallei , Coxiella burnetii , E. coli , P. aeruginosa or S. aureus , with an apparently unvaried morphology but exhibiting both a small size and a higher duplication time than their “normal” counterparts [ 90 , 194 , 196 , 198 , 199 ]. It has been proposed that these characteristics are related to the presence of a series of chromosomal mutations, leading to an altered electron-transport chain often resulting in menadione and/or hemin auxotrophy, or to the presence of thymidine auxotrophy [ 195 , 200 , 201 , 202 , 203 ], as well as other mutations, such as in the Pf1 prophage region [ 194 ]. Furthermore, the presence of genetic rearrangements as SCV inductors has also been described [ 204 ].…”