June 6, 1997 New Hyde Park, NY -- Penicillin-resistant pneumococci (PRSP) may be less virulent than penicillin-susceptible pneumococci (PSSP), according to a study conducted by Itzhak Levy, MD, and Lorry Rubin, MD, of the Pediatric Infectious Diseases Division of Schneider Children's Hospital of Long Island Jewish Medical Center.
The findings, which compare the ability of these two strains of pneumococci to cause disease, were presented by Dr. Levy at the 1997 Pediatric Academic Societies' Annual Meeting last month in Washington, DC.
Penicillin-resistant pneumococci exhibit less virulence than penicillin-susceptible pneumococci; animals injected with a resistant strain had more infections and a higher mortality rate.
The study found that animals injected with resistant bacteria did not necessarily develop more serious infection or outcome than animals injected with susceptible organisms. In fact, the latter experienced more serious illness and deaths than the former.
According to Drs. Levy and Rubin, pneumococci are the most common bacterial infections in infants and children and the prevalence of resistantpneumococci is increasing. It has been shown that one of the major risk factors for developing resistance is previous and repeated treatment with antibiotics.
It has also been shown that resistance to penicillin and related antibiotics is caused by alterations of the proteins that attach to the antibiotic. These proteins, in turn, produce the cell wall that is altered in resistant bacteria. The defective cell wall might explain the lower rate of serious infections seen in the animals.
"This theory is supported by a clinical study," said the specialists, "which showed that infections due to resistant pneumococci are not more severe than infections due to susceptible ones."
This led the investigators to question whether PRSP are more virulent than PSSP. In order to ensure the integrity and validity of the study, the resistant and susceptible organisms had to be the same for traits other than penicillin-resistance. In order to accomplish this, the resistant organism was derived from a susceptible strain by exposing it to an increasing concentration of antibiotic. According to the investigators, a great deal of time was spent developing the resistant strain from the susceptible in order to keep both strains as closely related as possible.
In order to compare the virulence, the animals were divided into two groups, one injected with the resistant strain and one with the susceptible strain. After one or two days, the animals injected with PSSP had more blood stream infections, a higher level of infection, and a higher mortality rate. Several groups of mice were studied over a six month period with similar results.
On the impact of the study, Drs. Levy and Rubin stated, "This preliminary data suggests that penicillin-resistant pneumococci may have less of an ability to cause disease than penicillin-susceptible ones."
Dr. Rubin is Chief of Pediatric Infectious Diseases and Professor of Pediatrics at the Albert Einstein College of Medicine. Dr. Levy is a Fellow in the same Division of Schneider Children's Hospital of Long Island Jewish Medical Center. The Hospital is well-known for its pediatric research programs.
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