الفهرس 
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Abstract
Lower respiratory tract infections are the major causes of morbidity due to many infectious agents. LRT infections cover three different clinical conditions: acute purulent bronchitis, exacerbation of chronic mucopurulent bronchitis, and pneumonia. The infection can be caused by virtually any type of infectious agent, however many risk factors were considered in developing of the LRT infections including advanced age, smoking, alcohol use, preexisting medical condition of serious illness, and others hospital related factors (4). LRT infections are remarkably simplified when the responsible pathogen is accurately defined. However, the frequent colonization with upper respiratory tract commensals is common and so the accurate diagnosis is essential to under stand epidemiology, pathogenesis and prevention of LRT infections (6). So the work of this study aimed to evaluate quantitative culture methods in diagnosing LRT infections in critically ill patients and to identify the bacterial, fungal, and M. pneumoniae isolates associated with LRT infections and determine their antibiogram. The present work included 67 selected critically ill patients admitted to the wards and ICUs of chest departments, of both Alexandria University and Alexandria Armed Forces Hospitals, over a period of 17 months starting November 1997 and ending in March 1999. All patients of the present study were evaluated as follow: 1. Complete history taking: 2. Laboratory investigations. 3. Radiological examination: Chest X-ray film. 4. Microbiological analysis: Sputum or endotracheal aspirates and B.BAL samples were taken from every patients, while for 23,and 30 patients out 67 studied patients, NB.BAL and PB.BAL samples were taken respectively. Every sample was subjected to: One) Direct microscopic examination: For each specimen two slides were prepared one for Gram stain and the other one for Ziehl Neelsen staining. Two) Graded Gram staining according to standard scales from 0 to 3+ was done according to the number of bacteria seen per high power field (22). Three) Direct culture on different cultures media. Four) Culture on diphasic Mycoplasma selective medium for all samples except sputum samples. Five) Quantitative culture using surface count method. The thresholds of ?105, ??104, and ? 103 cfu/ml were used as cut off for bacterial growth in sputum, B.BAL& NB.BAL and PB.BAL samples respectively (61, 104). - All isolates were identified by automated VITEK system except M. pneumoniae that was tested by diphasic culture medium and H. influenzae that was identified by satellitism test - All isolates except that of M. pneumoniae were tested for their antimicrobial susceptibility by disc diffusion method (109, 110). The results showed that: - 237 isolates were isolated related to 126 strains for 67 examined patients and they were belonging to 17 species of the microorganisms. - The organism with highest frequency of isolation between cases was Staph. aureus (34.3) followed by P. aeruginosa (32.8). While H. influenzae and Ent.