Staphylococcus Aureus colonization and bacteremia in children with cancer.

Staphylococcus aureus is an important cause of blood stream infections in children and adults. In recent years, new strains of methicillin-resistant S. aureus (MRSA), also known as community-acquired (CA)-MRSA have been isolated from otherwise healthy individuals. These strains frequently carry the Panton-Valentine Leukocidin (PVL) leukotoxin and belong to spa type 8; USA300 genotype, which is the most common strain causing CA-MRSA infections in the U.S. The clinical course of infections with PVL-positive S.aureus strains appears to be more severe than infection from PVL-negative strains. Bacteremia due to these strains has been reported in children, adolescents and adults. It is not known if methcillin-resistance or infection with CA-MRSA strains adversely affects outcome in children with cancer. Colonization of MRSA in healthy children has increased significantly since 2001 and has shown to be a risk factor for subsequent infection. However the prevalence of nasal and rectal colonization with MRSA; in particular with PVL-positive strains and the relationship between colonization and infection is not known in children or adults with cancer.

The epidemiology of MRSA and methicillin-sensitive S.aureus (MSSA) bacteremia and prevalence of MRSA nasal and rectal colonization in children with cancer was retrospectively studied from 2000 to 2007. Medical record review included patient demographics, underlying disease and antimicrobial susceptibility patterns of the MRSA and MSSA bacteremia isolates. Molecular typing was performed by polymerase chain reaction (PCR) on all isolates for detection of the PVL genes. Staphylococcus cassette chromosome (SCC) mec and spa typing was performed on all PVL-positive MRSA and MSSA bacteremia isolates and MRSA isolates causing colonization and infection. Demographic and treatment variables were compared between patients with MRSA and MSSA bacteremia and patients with PVL-positive and PVL-negative MRSA and MSSA bacteremia using exact two-sample Wilcoxon rank sum test or robust rank sum test for unequal variances and Fisher's exact chi-square test. The trend of MRSA/MSSA bacteremia and MRSA colonization was evaluated by logistic regression models.

Ten (19%) MRSA and 42 (81%) MSSA isolates from clinically distinct infectious bacteremic episodes were collected from 52 patients with cancer during the eight year study period. The proportion of cancer patients with MRSA, or MSSA bacteremia did not change significantly over the duration of the study. A third of the patients, 17 (33%), had complications. Thirty-eight (73%) of the bacteremic episodes were catheter-related. Catheters were removed significantly more often for MRSA infections than for MSSA infections for persistently positive blood cultures or complications (p=0.003). Subcutaneous ports was removed significantly more often than Hickman catheters (p= 0.005). The number of patients with persistently positive MRSA bacteremia were higher as compared to MSSA bacteremia (p= 0.004). Methicillin resistance was associated with decreased susceptibility to erythromycin (p=0.0003) and gentamicin (p=0.03). The difference in PVL positivity between MRSA and MSSA was statistically significant (P=0.01). None of the other variables studied including complications were significantly different between patients with MRSA or MSSA bacteremia or between patients with PVL-positive and PVL-negative S.aureus bacteremia. The number of patients colonized with MRSA compared to the total number of samples tested increased significantly between 2000-2001 and 2006-2007 (p=0.0007). PVL-positivity was associated both with increased colonization (p=0.004) and with an increased risk of infection (p=0.0005). There was a discordance in the spa types between the colonization and infection isolates.

This report represents the first description of the epidemiology of S.aureus bacteremia and colonization including analysis of PVL-positive strains in children with cancer. Methicillin-resistance or PVL-positivity did not appear to be associated with a worse outcome in our patient population. The number of patients colonized with MRSA increased significantly during this time period. PVL-positivity was associated with an increased risk of colonization and infection. Further studies are needed to confirm these observations in patients with cancer.

No relevant conflicts of interest to declare.