Experiment 04Culture Techniques • B.Sc. / M.Sc.
Pure Culture Isolation by Quadrant Streak Plate & Pour Plate Dilution
🎯 Aim: To isolate discrete, axenic single colonies from a mixed bacterial suspension (Serratia marcescens and Micrococcus luteus) using the quadrant streak plate technique, calculate colony-forming units (CFU/mL) by pour plate dilution, and record colonial morphological characteristics.
🔬 Theoretical Principle & Mechanical Dilution
Natural habitats contain complex poly-microbial communities. Physiological, biochemical, and genetic characterization requires the isolation of an axenic pure culture derived from a single ancestral cell:
- The Colony-Forming Unit (CFU) Concept: When individual bacterial cells are deposited spatially separated from one another onto a solid agar surface, each viable cell divides by binary fission ($1 to 2 to 4 to 8 dots$) over 18–24 hours to produce a macroscopic, genetically identical cluster of $approx 10^7–10^9$ cells termed a colony.
- Quadrant Streaking Mechanics: By progressively dragging an inoculating loop back and forth across four contiguous quadrants of an agar plate, and flaming the loop between each quadrant, cell density is mechanically diluted by a factor of $10^4–10^5$. In the final quadrant, single cells deposit far enough apart to form isolated, well-separated colonies.
- Colony Morphology Metrics: Form (circular, irregular, rhizoid); Elevation (flat, raised, convex, umbonate); Margin (entire, undulate, filiform); Pigmentation (prodigiosin red: S. marcescens; carotenoid yellow: M. luteus); Opacity (opaque, translucent).
Figure 4.1: Quadrant Streak Plate Mechanical Dilution Pattern & Single CFU Isolation
💬 Viva-Voce Questions & Model Answers
Q1: Why must the inoculating loop be flamed and cooled between streaking each quadrant?
Flaming incinerates all residual bacteria remaining on the wire loop from the preceding quadrant. Cooling prevents heat-killing of fresh cells. When the sterile loop is pulled across the previous quadrant, it picks up only a fraction of cells, achieving genuine step-wise mechanical dilution. Failure to flame produces confluent growth in all quadrants.
Q2: Why are petri plates always incubated in an inverted (upside down) orientation?
Water vapor evaporates from the warm agar during incubation and condenses on the cooler petri dish lid. If incubated right-side up, condensation droplets fall directly onto the agar surface, washing spreading bacteria across the plate and turning discrete colonies into confluent smears. Inverting the dish prevents moisture from dripping onto the agar.
Q3: What is the difference between a streak plate and a spread plate?
A streak plate uses an inoculating loop to mechanically dilute unknown cultures across solid agar for qualitative pure colony isolation. A spread plate uses an L-shaped glass rod (“hockey stick”) to evenly distribute a known volume of serially diluted liquid culture ($0.1,text{mL}$) across the entire plate for quantitative enumeration of viable microbial density (CFU/mL).
