Manual of basic techniques for a health laboratory - libdoc.who.int

5. Bacteriology 1975. Bacteriology5.1 IntroductionDirect microscopic examination of smears is generally not sufficient to identify abacterial species; precise identification can only be obtained by culture. The collectionand dispatch of specimens to referral laboratories is, therefore, of utmost importance.Nevertheless, direct microscopic examination of stained smears is anefficient way of studying the presence of bacteria in biological fluids that are normallysterile and in specimens from other sources. It may provide information ofgreat value for the diagnosis, immediate treatment and control of the disease. Forexample:● Specimens from male patients with urethritis at an early stage can be used todiagnose gonococcal infection with reasonable certainty (in females it is muchmore difficult).● Microscopic examination of sputum smears is a practical and effective techniquefor the detection of infectious cases of tuberculosis.● Microscopic examination of CSF is used in identifying the bacteria or fungi thatcause meningitis (see section 8.3.3).The diagnosis of some diseases is also possible through serology; an example issyphilis (see section 11.10). Serological techniques are also important for epidemiologicalsurveillance and early detection of diseases caused by bacteria that aredifficult to culture (e.g. Mycobacterium tuberculosis).5.2 Preparation and fixation of smears5.2.1 PrincipleThe sample to be examined (pus, sputum, urine centrifugate, CSF, etc.) is preparedas follows:● The specimen is spread in a thin layer on a glass slide.● It is allowed to dry completely.● It is fixed with 70% methanol or by heating before being stained.5.2.2 Materials and reagents●●●●●●Inoculating loop: this is a metal wire (usually made of nickel–chromium alloy)fixed on to a handle and bent into a loop at the other end. Make the loop withforceps, taking care that it is centred (Fig. 5.1). The actual diameter of the loopshould be 2mm.MicroscopeMicroscope slidesCoverslipsBunsen burner or spirit lamp70% Methanol.197