Bacterial Colony Morphology

A single clone of bacteria multiplying in the presence of nutrients to form a visible growth on the solid medium is called as a bacterial colony. Colony morphology is one among the various characters of bacteria which is also unique to a particular genus of bacteria that could be instrumental in the preliminary identification. Size (measured in millimetres-pin point (≤1mm), small (2-3mm) , medium (4-5mm) and large (>5mm) colonies, shape (circular, irregular, rhomboid, umbonate, umbonate, filamentous, rhizoid), odour (curdy, fruity,pungent, buttery, bleachy, putrid), texture (smooth, transparent, opaque, translucent, metallic sheen, glistening, rough, wrinkled, moist, mucoid, pitting, corroding) elevation over the solid medium (flat, raised,

After stirring/mixing the colonies for about 60 seconds we should look for the appearance of viscous nature and in that case the loop should be raised very slowly to see the formation of stringiness indicating that it is a gram negative bacterium [Figure 3]. Gram positive bacteria don’t form viscous emulsion and are negative for string like formation. Isolated bacterium was found to be negative for string test and was resistant to vancomycin 5 µg and sensitive to colistin 10 µg. Biochemically the bacterium was observed to be catalase positive, Indole, urease and citrate negative and showed no reaction/inert in tripple sugar iron agar (TSI) indicating that the organism has neither the ability to metabolise sugar nor the peptone. Antimicrobial susceptibility pattern of the isolated bacterium revealed sensitivity to imipenem, ciprofloxacin, ofloxacin, amikacin, piperacillin-tazobactum and cephalosporins including ceftriaxone, cefotaxime, ceftazidime, cefipime, cefipime-tazobactim and resistant to trimethoprim-sulfamethox azole. The isolated bacterium was grouped as belonging to diphteroids based on conventional methods and was not speciated due to the unavailability of advanced identification system that include molecular

Although the colony on the whole appeared normal, the cells lining the colony surface showed donut shape. This study has concluded that the donut shape of cells is an intermediary morphological form between the young spiral forms and the older coccoid foms [9]. Another previous report has hypothesized that under stress (heat, freezing, radiation, exposure to chlorine and copper) bacteria like Campylobacter spp, Yersinia enterocolita which are bacilli could produce donut cells and that this phenomenon might also be seen among Staphylococcus spp [10]. Streptococcus pneumoniae colonies were also previously reported to produce donut shaped colonies on blood agar isolated from throat swab [11]. Bacteria have been noted to respond to the external stimuli/variable environmental conditions that include immune response, physicial stress, antibiotic stress and nutritional deficiencies by acquiring survival strategies in the form of altering cell morphologies (curved/rod-filamentous), biofilm production, small colony variants and quorum sensing [12, 1, 13,14 and