الفهرس 
Acknowledgement
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Abstract
Virtually every surface examined in natural, industrial and pathogenic ecosystems is colonized by biofilms consisting of adherent (sessile) bacterial populations enmeshed within a glycocalyx matrix. Establishment of these consortia heralds the onset of infections which readily become chronic, despite antibiotic treatment regimens. Since biofilm is not extensively addressed in literature, compared to the planktonic mode of microbial growth, the current study concentrates on the biofilm as a phenomenon responsible for many of the bacterial activities in health related and natural settings.
The method generally applied for the screening of biofilm formation was that using the glass cover slip. A variation of this method, with further optimization, was used when biofilm formation was tested in eppendorfs, membrane filters and some microtiter plate experiments.
For the vast investigation of the action of antimicrobial agents on biofilm formation or eradication of formed biofilms, a colorimetric method was applied.
The current study comprised five standard strains and forty nine clinical isolates obtained from blood, wound, urine, skin and sputum, with no direct correlation between the source of the isolate and the strength of the biofilm it formed. The microorganisms used were collectively of the staphylococcus, pseudomonas, klebsiella, candida, enterococcus, proteus and E. coli groups and their biofilm counts were determined. Generally speaking, pseudomonas and staphylococcus formed the strongest biofilms and were therefore used to study some factors believed to play a role in biofilm formation.
When comparative biofilm formation by representative strains was studied in/on eppendorfs, membrane filters, microtiter plates and glass covers, P11 and S5 formed strong biofilms, while Kp formed weak ones except in eppendorf. It was also observed that membrane filters supported the highest pseudomonas and staphylococcus biofilms.
Several factors were considered in the current study. Upon investigating the effect of time (short and long periods) on biofilm formation on glass, it was shown that P11 biofilm formation reached a maximum after six hrs and that high count was maintained till 24 hrs. Study of biofilm formation over a longer period of time on glass and other substrates revealed that P11 biofilm count was almost maintained over the 6 days of the experiment on all substrates. S5 behaved almost similarly in eppendorf and microtiter plate, whereas on membrane filter, the count of the biofilm increased with time. Kp almost always formed weak biofilms. A conclusion was therefore drawn, based on the results of this experiment and another one conducted over two weeks that a 24 hr period was enough time to study biofilm formation, except in the cases where old biofilms were required.
Alginate determination was carried out in four pseudomonas biofilms, it was noted that maximum alginate production always occurred during the first 24 to 48 hrs of biofilm formation which was, in most cases, consistent with the count of the biofilm cells. At the end of the two week examination period, alginate production mostly decreased.
The effect of the culture medium on biofilm formation by thirty three microorganisms was also tested. Little effect was associated with the difference in culture media in most of the cases, with the exception of the selective media that mostly resulted in the formation of weaker biofilms. This was more evident in case of the pseudomonads compared to the staphylococci under test. These findings led to the use of N.B. as routine growth and biofilm formation medium in this study.