Relationship between the inoculum dose of Streptococcus pneumoniae and pneumonia onset in a rabbit model.

It is generally assumed that the development of bacterial pneumonia becomes possible when the dose of inhaled or aspirated pathogens overwhelms the respiratory tract host defence system, but this hypothesis has not yet been tested either clinically or experimentally. This study evaluated inoculum dose in relation to onset of experimental pneumococcal pneumonia, and estimated the median effective dose resulting in pneumonia in healthy New Zealand White rabbits (mean+/-sd 4.75+/-0.25 kg (n = 27)). Rabbits were endobronchially inoculated with increasing doses of Streptococcus pneumoniae and pneumonia onset was observed over the following 96 h. The diagnostic approach was based on the Clinical Pulmonary Infection Score, modified for use in rabbits. Inoculation of S. pneumoniae at doses of >4.60 log(10) cfu made the development of pneumonia in rabbits more predictable (up to 90%). Lower doses of bacteria failed to cause pneumonia in 80% of inoculated animals. The median effective dose was estimated by means of logistical regression, probit analyses and the Reed-Muench method, and corresponded to 4.32, 4.38 and 4.67 log(10) cfu, respectively. It is speculated that development of pneumococcal pneumonia becomes more likely when the inoculum dose exceeds a threshold of antibacterial protection, making inoculum dose a risk factor for disease onset.

Antimicrobial efficacy of external fixator pins coated with a lipid stabilized hydroxyapatite/chlorhexidine complex to prevent pin tract infection in a goat model.

BACKGROUND: Pin tract infection is a common complication of external fixation. An antiinfective external fixator pin might help to reduce the incidence of pin tract infection and improve pin fixation. METHODS: Stainless steel and titanium external fixator pins, with and without a lipid stabilized hydroxyapatite/chlorhexidine coating, were evaluated in a goat model. Two pins contaminated with an identifiable Staphylococcus aureus strain were inserted into each tibia of 12 goats. The pin sites were examined daily. On day 14, the animals were killed, and the pin tips cultured. Insertion and extraction torques were measured. RESULTS: Infection developed in 100% of uncoated pins, whereas coated pins demonstrated 4.2% infected, 12.5% colonized, and the remainder, 83.3%, had no growth (p < 0.01). Pin coating decreased the percent loss of fixation torque over uncoated pins (p = 0.04). CONCLUSION: These results demonstrate that the lipid stabilized hydroxyapatite/chlorhexidine coating was successful in decreasing infection and improving fixation of external fixator pins.