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INCUBATION OF PLATES 4.1. Label the plates and incubate at 28 +/- 20 C for 3 to 5 days for fast growing Rhizobia and 5 to 10 days for slow growing ones. 4.2. Colony Counting aids; Count the colonies with the aid of magnifying lens under uniform and properly controlled artificial illumination. Use a colony counter, equipped with a guide plate and rules in centimeter square. Record the total number of colonies with the hand tally. Avoid mistaking particles of un-dissolved [Act No.6 of 2006] 39 medium or precipitated matter in plates for pin-point colonies. To distinguish colonies form dirt, specks and other foreign matter, examine doubtful objects carefully. 4.3. Count all plates but consider for the purpose of calculation plates showing more than 30 and less than 300 colonies per plate. Disregard colonies which absorb Congo red and stand out as reddish colonies. Rhizobium stands out as white, translucent, glistening and elevated colonies. Count such colony numbers and calculate figures in terms of per litre, of carrier. Also check for freedom from contamination at 108 dilution.