Microbiology with Lab: A Fundamental Approach (D311)

Correct Answer

B. It slows down metabolic processes and preserves the culture.

Explanation

Storing microbial cultures at 4 degrees Celsius slows down the metabolic activities of microorganisms, which helps in preserving their viability for future observations. This low temperature reduces the rate of growth and reproduction, preventing the culture from degrading or growing uncontrollably. By maintaining cultures at this temperature, microbiologists can prolong the life of the culture and conduct experiments at a later time without the culture becoming overgrown or contaminated.

Why other options are wrong

A. It promotes rapid growth of microorganisms.

Storing cultures at 4 degrees Celsius actually slows down, not promotes, growth. The low temperature prevents microorganisms from growing too quickly, ensuring the culture remains stable for future use.

C. It prevents contamination from airborne microbes.

While proper storage techniques, such as sealing cultures in sterile containers, can help prevent contamination, the primary reason for storing cultures at 4°C is to preserve them by slowing their metabolism. It does not directly prevent airborne contamination.

D. It enhances the visibility of the cultures under a microscope.

Storing cultures at 4°C does not directly affect their visibility under the microscope. While it preserves the cultures for future use, visibility is typically influenced by staining methods and microscope adjustments, not temperature storage.

Correct Answer

C. In the broken glass container

Explanation

Broken glass slides pose a physical hazard and should never be disposed of in regular trash or biohazard bins unless contaminated. Microbiology labs typically have a designated broken glass container that ensures safe disposal and prevents injuries. If the broken glass has been exposed to bio hazardous material, it may need to be decontaminated before disposal, following lab safety protocols.

Why other options are wrong

A. In the regular trash

This is incorrect because broken glass is a safety hazard. Disposing of it in regular trash increases the risk of injury to lab personnel and waste handlers.

B. In the biohazard waste container

This is incorrect because biohazard waste containers are intended for contaminated materials such as used cultures, gloves, and petri dishes. Broken glass should go into a dedicated container unless it is contaminated, in which case additional decontamination steps are necessary.

D. In the recycling bin

This is incorrect because lab glassware, especially if contaminated, is not suitable for recycling due to the risk of pathogen exposure and potential chemical contamination.

Correct Answer

C. Leaving personal items on the lab bench

Explanation

Leaving personal items on the lab bench is not a necessary cleanliness task and is actually a violation of lab safety protocols. Personal items can become contaminated with microorganisms and may spread contaminants outside the lab environment. Best practices require that all personal belongings, such as bags and phones, be kept away from the lab bench to maintain a sterile working area.

Why other options are wrong

A. Disposing of used petri dishes in the designated biohazard container

This is incorrect because proper disposal of biohazardous waste is essential to prevent contamination and ensure laboratory safety. Petri dishes with microbial cultures should always be disposed of in designated biohazard containers.

B. Wiping down the work surface with disinfectant

This is incorrect because disinfecting the work surface helps remove any residual contaminants and reduces the risk of spreading microorganisms. It is a critical step after finishing lab work.

D. Washing hands thoroughly with soap and water

This is incorrect because handwashing is one of the most important hygiene practices to prevent the spread of microbes and contamination after handling microorganisms in the lab.

Correct Answer C. It does not contain agar.

Explanation

The defining characteristic of liquid media is that it does not contain agar, which distinguishes it from solid media. Liquid media is commonly used to grow bacteria in a free-floating state, providing a uniform environment for bacterial growth without the solidifying properties of agar. This media is ideal for culturing microorganisms in large quantities or for biochemical testing.

Why other options are wrong

A. It contains agar as a solidifying agent.

This statement describes solid media, not liquid media. Solid media contains agar, which helps to solidify the medium and create a surface for bacterial growth.

B. It is used exclusively for growing fungi.

Liquid media is not exclusive to fungi; it is used to grow a wide range of microorganisms, including bacteria. Fungi can also be cultured in liquid media, but it is not specific to them.

D. It is always colored blue.

The color of liquid media is not universally blue. Liquid media can come in various colors depending on the components and indicators used, but the color is not a defining characteristic.

Correct Answer A. By competing for nutrients and space

Explanation

Microbiota, or the normal flora of microorganisms, play a protective role in preventing pathogenic organisms from colonizing the human body. One of the primary mechanisms they use is competition for resources, such as nutrients and space. By occupying ecological niches in the body, normal microbiota can limit the ability of pathogens to establish themselves, essentially outcompeting harmful microbes for the available resources.

Why other options are wrong

B. By producing harmful toxins

This is incorrect. While some bacteria in the microbiota may produce toxins, this is not typically how microbiota prevent colonization of pathogens. The presence of microbiota mainly prevents pathogen colonization through competition, not by producing harmful substances.

C. By increasing the pH of the skin

This is incorrect. The pH of the skin may be influenced by the microbiota, but it is not the main mechanism by which they prevent pathogen colonization. The primary method of prevention is through competition for space and resources.

D. By directly attacking pathogens

This is incorrect. While some microbiota may produce antimicrobial compounds, their primary defense mechanism is through competition for resources, not direct attack of pathogens. The body’s immune system is more responsible for actively attacking pathogens.

Correct Answer

B. To reduce the refraction of light

Explanation

Immersion oil is used with the 100X objective lens to reduce the refraction of light as it passes from the specimen to the lens. Air between the slide and the objective lens causes light to bend, leading to a loss of resolution and clarity. Immersion oil has a similar refractive index to glass, which minimizes light bending and improves the resolution and clarity of the image.

Why other options are wrong

A. To enhance color contrast

Immersion oil does not directly enhance color contrast. Its primary function is to improve resolution by reducing refraction, not altering the color of the image.

C. To increase the magnification

Immersion oil does not increase magnification. The 100X objective lens itself provides the magnification. The oil helps to enhance the image quality by improving resolution, but it does not change the magnification power.

D. To prevent lens fogging

Immersion oil is not used to prevent lens fogging. Fogging is typically caused by condensation or environmental factors, while immersion oil addresses issues related to light refraction to enhance clarity.

Correct Answer

B. To create a uniform suspension for microscopic examination

Explanation

Mixing a solid culture with a drop of water ensures that the bacteria are evenly distributed in the sample, making it easier to observe individual cells under a microscope. This step helps avoid clumping, allowing for clearer visualization of the bacterial morphology and behavior. A uniform suspension also facilitates more accurate counting and identification of bacteria.

Why other options are wrong

A. To enhance the color of the culture

This is incorrect because mixing with water does not change the color of the culture. The purpose is not related to enhancing color but to prepare the sample for better observation under the microscope.

C. To increase the volume of the sample

This is not the primary reason. While adding water may slightly increase the volume, the main reason for mixing water with the culture is to create a suspension for easier microscopic examination, not just to increase the sample volume.

D. To prevent contamination from the environment

This is incorrect because the purpose of adding water is not related to preventing contamination. Contamination is controlled through sterile techniques, not by the addition of water to the culture.

Correct Answer

A. Microbes in quadrant 3 have more space and nutrients

Explanation

In streak plate techniques, as the streaking progresses from quadrant 1 to quadrant 3, the bacteria are increasingly diluted, leading to fewer cells in quadrant 3. The cells in quadrant 3 thus have more space and nutrients, allowing them to grow into larger colonies compared to quadrant 1. In quadrant 1, the concentration of bacteria is higher, leading to smaller colonies because the available nutrients are being consumed quickly.

Why other options are wrong

B. Microbes in quadrant 1 are older

This is incorrect because the age of the microbes does not significantly impact the size of the colonies in the context of streaking. The size of the colonies is primarily determined by the available nutrients and space, not by the age of the microorganisms.

C. Quadrant 3 has a different temperature

This is incorrect because, in a typical streaking process, the temperature remains constant throughout the plate. Variations in temperature could affect colony growth, but this is not the primary reason for the size difference between the quadrants.

D. Quadrant 1 has more contaminants

This is not correct because quadrant 1 contains the initial inoculum, which typically has a higher density of bacteria. The colony size difference is due to the dilution of bacteria in subsequent quadrants, not because of contamination.

Correct Answer

B. To ensure the slide is clean for future use

Explanation

It is important to wipe off any oil from prepared stock slides to ensure that the slide remains clean and free from any residue that could interfere with future observations. Oil left on the slide can degrade over time and make it difficult to examine under the microscope. By cleaning the slide thoroughly, it is ready for use in subsequent experiments without the risk of contamination or the buildup of residue.

Why other options are wrong

A. To prevent contamination of other slides

This is not the primary reason for wiping off oil. While it is true that oil can contaminate other slides if transferred, the main reason for cleaning is to ensure that the slide remains suitable for future use and observation.

C. To avoid damaging the microscope

This is incorrect because oil typically affects the slide, not the microscope directly. However, oil on the objective lens, if not cleaned properly, could potentially damage the lens. The primary concern with wiping oil off the slide is to maintain its cleanliness for future use.

D. To make the slide easier to handle

This is not the primary reason either. While cleaning the slide might make it easier to handle, the primary concern is to ensure that the slide is free of oil and ready for use in future experiments without the potential for contamination or buildup of residue.