LAB FINAL Review

You should begin viewing a specimen with what objective lens?

4X As a general rule of microscopy, you should use the lowest-power objective to begin viewing a specimen. Once the image is in focus, you can switch to a higher-powered objective. One of the main advantages of the low-powered objectives is their large field of view. This makes it easy for you to quickly survey the contents of your slide.

If you view a specimen at 40X total magnification with a 10X ocular lens, then you are viewing the specimen with what objective?

4X Total magnification is calculated by multiplying the magnifying power of the objective lens by the magnifying power of the ocular lens: 4X objective x 10X ocular lens = 40X total magnification

After you add crystal violet and rinse the slide, what should be your next step?

Add Gram's iodine. Gram's iodine serves as a mordant. Notice in the image below how it forms an insoluble complex with crystal violet. This helps the crystal violet remain in the gram-positive cells during decolorization.

What is the best procedure for decolorization?

Add decolorizing agent until run-off is clear. This method allows the decolorizing agent to dissolve the outer membrane of Gram-negative cells and rinse out the crystal violet from the thin layers of peptidoglycan. This causes the run-off to be purple. When that purple color ceases to appear in the run-off, decolorization should be stopped so as not to also cause the Gram-positive cells to lose their purple color. Watch the video to review the process of decolorization.

After Gram's iodine is added and then rinsed, what is the next step in the Gram stain procedure?

Add decolorizing agent. The decolorizing step removes the crystal violet from the gram-negative cell walls, but not the gram-positive walls. Watch the video to review the addition of decolorizing agent.

While looking through a microscope at 400X total magnification, you see this image. To improve the quality of this image, what should you do?

Adjust the condenser aperture diaphragm. The specimen is in focus, but the contrast is poor. You can enhance contrast by moving the aperture diaphragm toward the closed position. This restricts the angles at which light strikes the specimen and thereby tends to eliminate the bright, washed-out appearance of the image. This is a better alternative than reducing lamp intensity. Reducing the lamp intensity would make the image less bright, but contrast would remain poor.

You have just rotated the 40X objective into position after viewing the specimen with the 10X objective. What would your next step most likely be?

Adjust the fine focus knob. After changing objectives, you should always check to make sure your image is as well focused as possible. Your objectives are most likely parfocal, so not much adjustment should be necessary. The working distance between the 40X objective and the specimen is very small, so adjusting the fine focus (rather than the coarse focus) will help avoid smashing the objective into the slide.

If you FORGOT to do the decolorizing step, what colors would the Gram-positive and Gram-negative cells be when viewed at the end of the procedure?

All would be purple. If the decolorization step were omitted, both types of cell walls would retain the primary stain, which is crystal violet. This micrograph shows Gram-positive cocci and Gram-negative bacilli that were Gram stained with the decolorizing step omitted. Note that both are the same color.

Two specimens, A and B, were inoculated into Urea broth. What can you conclude from your observations? Based on your observation of the pictured Urease tests, select ALL appropriate statements.

Bacterium A converted urea to ammonia Bacterium A increased the pH of the medium Bacterium B decreased the pH of the medium Bacterium B does not produce urease

You are doing research on wetland water samples and come across this microscopic organism. Assuming this organism is a protozoan, how would you classify it?

Ciliate This organism, Vorticella, is a ciliate. Numerous cilia surround its cytostome. In contrast to Paramecium, this protozoan has no cilia on the remainder of its body. Vorticella is less motile than Paramecium, spending much of its life anchored to substrate via a long retractable stalk.

Which of the motility groups is represented in this micrograph?

Ciliates The micrograph shows a member of Paramecium, which is a ciliate. The ciliates use cilia to move around, and in the case of Paramecium, the entire cell surface is covered with these cilia. Other ciliates have a more limited arrangement of cilia.

When you look through the ocular lenses, you see this image of the endospore-forming bacteria Bacillus anthracis under oil. Which of the following qualities is most lacking in this image?

Contrast Contrast, which is lacking in this image, refers to the difference in light intensity between the specimen's features and between the specimen and the background. You can enhance contrast by adjusting the lamp intensity dial and the aperture diaphragm on the condenser.

Use the micrograph of the Gram stain to draw conclusions and make predictions about the indicated bacterium. Based on your observation of the indicated bacterium in this Gram stain, select ALL appropriate statements.

Eosin Methylene Blue (EMB) or MacConkey agar would NOT be good choices of media to grow the indicated bacterium The indicated bacterium is Gram-positive The indicated bacterium has a strepto- formation The indicated bacterium is a bacillus

Identify the unknown organism from the following information:

Escherichia coli Escherichia coli is the only organism that matches these test results. E. coli are gram-negative bacilli (red rods on a Gram stain), which produce acid with gas for glucose and lactose fermentation (yellow with a bubble), are positive for the indole and methyl red tests (red colors), and are negative on the Voges-Proskauer and urease tests (non-red color). The next closest possibilities are Enterobacter aerogenes and Proteus vulgaris, but each of these species is inconsistent with three different test results.

The image below shows the results of fermentation tubes inoculated with an unknown microbe. Based on the results and the ID matrix below, which of the following microbes could this unknown be?

Escherichia coli The tubes show acid with gas for all sugar fermentations. Escherichia coli produces acid with gas in both the glucose and lactose tests. E. coli fermentation of sucrose is inconsistent (hence, the "Variable"), so acid with gas is a possibility. Two organisms in the ID matrix (Enterobacter aerogenes & E. coli) match the results shown, but only E. coli is listed as a possible answer choice.

The following observations, and the ID matix below, suggest that your unknown microbe is one of two possible species. Which two?

Escherichia coli or Proteus vulgaris The recorded observations are consistent with both Escherichia coli and Proteus vulgaris. Both are gram-negative bacilli that have the same expected results for glucose fermentation and the indole test. In contrast, the recorded indole result is not consistent with either of the other answer choices offered—Enterobacter aerogenes or Pseudomonas aeruginosa.

Which structure is indicated by the arrow in this micrograph of Euglena?

Eyespot The eyespot is often seen as a red spot toward the anterior end of the Euglena. It is a sensory organ that detects light intensity. Protozoa are aquatic and the light intensity varies at different depths in the environment. As a result of sensory information, the Euglena can change its depth to enhance photosynthesis and find food.

To which motility-based group does this protozoan belong?

Flagellates This organism, Leishmania, is a parasitic flagellate that causes leishmaniasis. The flagellated promastigotes live in the digestive system of sand flies, and are transmitted to humans and other mammals when the insect feeds. In cutaneous leishmaniasis, these bites become skin lesions that eventually leave permanent scars.

What structure is used for motility in Euglena?

Flagellum Euglena is part of the motility-based group known as the flagellates. These organisms use one or more flagella to move.

Sort each of these characteristics as either belonging to Gram-positive or Gram-negative bacteria.

Gram + CV retained during decolorization Many layers of peptidoglycan Stain purple by gram stain Gram - CV NOT retained during decolorization 1 or 2 layers of peptidoglycan Stain red or pink by gram stain

If you were staining Gram-positive bacilli and Gram-negative cocci, which of these micrographs would correctly represent the appearance of these cells after staining with crystal violet, rinsing, and blotting?

If you view the slide after the crystal violet step, you will have performed a simple stain on this smear. All cells would stain purple, as you will see in this video. The decolorization step is the step that rinses the crystal violet from the Gram-negative cell wall, but not the Gram-positive. This allows the differential staining of the Gram-negative cell wall with safranin.

From your first day of observations, your unknown bacteria appear to be gram-positive cocci that produce yellow/golden growth on TSA. Based on these first few observations, and the ID matrix below, the unknown is either Micrococcus luteus or Staphylococcus aureus. Which of the following tests would NOT distinguish these two species?

Indole Both Micrococcus luteus and Staphylococcus aureus give a negative result for the indole test. This means the indole test result will NOT be useful in differentiating the two species. In contrast, any of the other tests listed—methyl red, urease, or glucose fermentation—will yield different results for M. luteus versus S. aureus.

Which of the following tests would be useful in distinguishing Escherichia coli from Pseudomonas aeruginosa?

Indole Escherichia coli and Pseudomonas aeruginosa give different results on the indole test. Therefore, this test can be used to distinguish the two species. In contrast, both species produce negative results for the Voges-Proskauer and urease tests. And both species could produce the same result for sucrose fermentation, since the expected E. coli result is "variable"—meaning anything is possible. Tests that have the same expected result for each organism are not useful in distinguishing those organisms

Kelsey inoculated her unknown bacterial sample onto SIM agar and obtained the pictured result. She described her sample as indole negative and hydrogen sulfide (H2S) positive. Unfortunately, Kelsey lost points for her description of her SIM test results. Curious by nature, she turned to you to ask if you could help her determine why she did not earn full credit for her work. sed on your observation of Kelsey's SIM test, select ALL of the reasons she lost points.

Kelsey did not address the motility of the organism As evidenced by the lack of an aqueous layer on the top of the medium, Kelsey forgot to add Kovacs' reagent. As such, she could not truly make a conclusion about whether or not indole was produced

Select the appropriate statements regarding the Voges-Proskauer (VP) test Select ALL correct statements.

MRVP media is inoculated using an inoculating loop. The media used for the VP test contains glucose (dextrose). The VP test is part of the IMViC panel of tests. To develop the Voges-Proskauer test, alpha-naphthol must be added followed by potassium hydroxide.

When comparing a wet mount preparation to a commercially prepared slide, which of the following characteristics of Protozoa is best observed using a wet mount slide preparation?

Mechanism of motility Wet mount preparations are useful for observing motility because the cells are still alive and can move around. In contrast, specimens on a prepared slide are dead. Both types of slides have their uses, though. For example, when working with infectious protozoa, a prepared slide is safer to work with than a wet mount.

What is the purpose of methylcellulose when preparing a wet mount of protozoa?

Methylcellulose increases the viscosity of the mount. Methylcellulose is added to a wet mount to increase the viscosity of the fluid. The increased viscosity slows the motility of the protozoa, helping to keep them in your field of view longer. This is especially useful for fast-moving protozoa such as Euglena.

As bacterial cells age, their peptidoglycan begins to break apart. What would be the effect on decolorization?

Old Gram-positive cells will be decolorized. Normally, Gram-positive cells aren't decolorized because they have many layers of peptidoglycan that hold the crystal violet/iodine complex during decolorization. However, old Gram-positive cells have less intact peptidoglycan, and the crystal violet/iodine complex is washed out more easily. These cells are indicated by the arrow in this micrograph.

Identify the unknown organism using the following results table and the ID matrix below.

Pseudomonas aeruginosa Pseudomonas aeruginosa is the only organism that matches these test results. All of the other answer choices have two expected test results that are inconsistent with those shown in the results table.

Which structure is used for motility in the organism in the following micrograph?

Pseudopodium The organism in this micrograph is Entamoeba histolytica, an amoeba. Amoebas use pseudopodia for motility. A new pseudopodium is created by pushing the cytoplasm away from the center of the cell. As the cytoplasm moves in a particular direction, the cell membrane moves along with it. This pulls the rest of the cytoplasm as well, thus moving the entire amoeba.

After you add crystal violet to the slide, what should be your next step?

Rinse with distilled water. For each reagent to work effectively, it is important to rinse the slide with water after each reagent has been applied. Watch the video to see the effect of rinsing the slide with water. If there is a large pool of excess crystal violet on the slide, then the molecules of the next reagent (Gram's iodine) will encounter and adhere to those crystal violet molecules and be less likely to reach the crystal violet within the cell.

How does safranin affect Gram-positive cells?

Safranin penetrates the cell wall, but is masked by the darker crystal violet stain. In the Gram-positive cell walls, most of the spaces between the molecules that make up peptidoglycan are already occupied by crystal violet/iodine complexes.

Two different bacterial samples, A and B, were inoculated onto Blood agar. What conclusions can you make? Based on your observation of the pictured Blood agar plates, select ALL appropriate conclusions.

Specimen A exhibits beta hemolysis Specimen B exhibits gamma hemolysis

Two different bacterial samples, A and B, were inoculated into MR-VP (Methyl Red-Voges Proskauer) broth. The following image shows what the tubes looked like for the Methyl Red portion of the test. Select ALL appropriate statements regarding the pictured specimens that were subjected to the Methyl Red test.

Specimen A fermented glucose and formed low levels of acid end-products Specimen B fermented glucose and formed high levels of acid end-products Specimen B can likely convert glucose to lactate, formate, or acetate

Two different bacterial samples, A and B, were inoculated onto Starch agar. What can you conclude based on your observations? Based on your observation of the pictured specimens that are growing on the Starch agar plates, select ALL appropriate conclusions.

Specimen A is able to hydrolyze starch

Two different bacterial samples, A and B, were inoculated into the motility media shown here. Based on your observation of the pictured specimens select ALL appropriate statements.

Specimen A is nonmotile Specimen B is motile

Two different bacterial samples, A and B, were grown on Gelatin deeps. This is a picture of the test results. Select ALL appropriate statements regarding the pictured Gelatin test results.

Specimen A produced gelatinase Specimen A hydrolyzed gelatin Specimen B did not produce gelatinase

Two different bacterial samples, A and B, were inoculated onto SIM media. What can you conclude based on your observations? Based on your observation of the pictured specimens that were inoculated into SIM tubes, select ALL appropriate conclusions.

Specimen A produced indole (Detection of pink ring) Specimen A is motile (pink cloudiness surrounding stab line) Specimen B did not produce indole (no pink ring) Specimen B did not produce hydrogen sulfide (H2S) (No black precipitate)

Two different bacterial samples, A and B, were grown in Ornithine Decarboxylase media. The following image shows what the tubes looked like following 72 hours of incubation at 35 ± 2°C. Select ALL appropriate statements regarding the pictured specimens.

Specimen B converted the amino acid ornithine to the amine putrescine. Specimen B fermented glucose. Specimen A fermented glucose.

Two different bacterial samples, A and B, were analyzed with the Voges-Proskauer (VP) test. The results are pictured here. Select ALL appropriate statements regarding the pictured Voges-Proskauer test results.

Specimen B fermented glucose and formed acetoin (acetylmethylcarbinol). Specimen B fermented glucose and formed neutral end-products. Specimen B had a positive result for the VP test.

Based on the results of this Gram stain and the ID matrix below, which of the following microbes could this possibly be?

Staphylococcus aureus During the Gram stain, gram-positive microbes retain the crystal violet stain and appear purple/blue. These spherical cocci are gram-positive, which is consistent with Staphylococcus aureus. Micrococcus luteus is also listed in the ID matrix as gram-positive cocci and would give similar Gram stain results. However, Micrococcus luteus was not listed as a possible answer.

A Lactose Fermentation test was performed on two different bacterial samples, A and B. Based on your observations what can you conclude? Based on your observation of the pictured Lactose Fermentation tests, choose all appropriate statements.

Test organism A ferments lactose Test organism A produced gas Test organism A lowered the pH of the medium Test organism B does not ferment lactose

Assuming the Gram stain smear was blotted and observed following the decolorization step, which micrograph correctly illustrates how Gram-positive cocci and Gram-negative bacilli would appear?

The Gram-positive cells have thick walls, and they will retain the purple crystal violet stain if decolorization was carried out correctly. The Gram-negative cells will be decolorized and are colorless. Watch the video to review the effect of decolorization on both types of cells.

Select the appropriate statements regarding the Methyl Red (MR) test Select ALL correct statements.

The MR test is part of the IMViC panel of tests The media used for the MR test contains glucose (dextrose) The culture that is growing in the MRVP broth should be split into separate tubes or grown in separate tubes before adding the reagents for the MR portion of the test. MRVP media is inoculated using an inoculating loop.

Use the micrograph of the Gram stain to draw conclusions and make predictions about the pictured bacterium. Based on your observation of the Gram stained bacterial specimen, select ALL appropriate statements.

The bacteria pictured are cocci. The indicated bacterium is Gram-negative. The pictured bacteria produce endotoxin (also known as lipopolysaccharide, LPS).

Use the micrograph of the Gram stain to draw conclusions and make predictions about the indicated bacterium. Based on your observation of the indicated bacterium in this Gram stain, select ALL appropriate statements.

The bacteria pictured are cocci. The indicated bacterium is Gram-positive. The bacteria have a staph arrangement. Eosin Methylene Blue (EMB) or MacConkey agar would NOT be good choices of media to grow the indicated bacteria.

What can you conclude about the Gram-stained specimen? Based on your observation of this Gram-stained specimen, select ALL appropriate statements.

The bacteria would not have been heat fixed in the preparation of this slide The pictured bacteria are bacilli This is a picture of a negative stained specimen

What can you conclude about the Gram-stained specimen? Based on your observation of this Gram-stained specimen, select ALL appropriate statements.

The bacteria would not have been heat fixed in the preparation of this slide The pictured bacteria are cocci This is a picture of a negative stained specimen

What happens to the Gram-positive cell wall during decolorization?

The decolorizing agent dehydrates the peptidoglycan. Removing water from (or dehydrating) the peptidoglycan allows the decolorizing agent to shrink the spaces through which the crystal violet-iodine complexes might be able to pass. This makes it more difficult for the purple stain to be removed. Watch the video to view the effect of decolorization on the Gram-positive wall below.

What happens to the Gram-negative cell wall during decolorization?

The decolorizing agent dissolves the outer membrane. The decolorizing agent dissolves the outer membrane of Gram-negative cells. This allows the decolorizing agent to penetrate the thin layers of peptidoglycan and rinse away the crystal violet/iodine complexes. Watch the video to review the effect of decolorization on the Gram-negative cell wall.

A Catalase test was done on an unknown specimen. Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Catalase test result.

The image shows a positive test result. The organism tested is probably an aerobe or a facultative anaerobe. The organism tested is protected against oxygen radicals.

Use the micrograph of the Gram stain to draw conclusions and make predictions about the indicated bacterium. Based on your observation of the indicated bacterium in this Gram stain, select ALL appropriate statements.

The indicated bacterium is Gram-negative. The indicated bacterium is a bacillus. The indicated bacterium produces endotoxin (also known as lipopolysaccharide, LPS).

Based on your observation of the pictured Urea broth what could have led to the result? Based on your observation of the pictured Urea test, select all the possible situations that could have led to the observed result.

The media was incubated at the incorrect temperature. The test organism does not produce urease. The media was not incubated long enough. The media was incorrectly prepared. The media was not inoculated.

Observe the pictured Sucrose Fermentation test. Select the three best hypotheses that could explain why the broth in the Durham tube appears yellow while the rest of the broth appears red.

The organism is limited in its ability to ferment sucrose. Since diffusion is restricted within the Durham tube and it contains only a small volume of media, it turned yellow first. The sample was incubated less than 24 hours, which allowed for only limited fermentation. The test organism ferments sucrose best under reduced oxygen conditions.

A Catalase test was done on an unknown specimen. Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Catalase test result.

The organism produces catalase. The organism can convert hydrogen peroxide to water and oxygen.

An unknown specimen was grown on Mannitol Salt agar (MSA). Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Mannitol Salt agar (MSA) test result.

The organism tested ferments mannitol. The organism tested is able to grow in the presence of high salt. The organism tested is possibly a type of Staphylococci.

Two different bacterial samples, A and B, were inoculated into MR-VP (Methyl Red-Voges Proskauer) broth. The following image shows what the tubes looked like for the Methyl Red portion of the test. What could have led to the result for sample A? Select ALL possible scenarios.

The person conducting the test forgot to add Methyl Red to tube A Specimen A fermented glucose and formed low levels of acid end-products The media was prepared incorrectly The test was conducted less than 24 hours following inoculation

What can you conclude about the pictured bacteria? Based on your observation of this stained specimen, select ALL appropriate statements.

The pictured bacteria are bacilli The indicated structure is an endospore If a Gram stain was performed on this bacterium within 24 hrs of a fresh culture, it would most likely be Gram-positive.

What can you conclude about the Gram-stained specimen? Based on your observation of this Gram-stained specimen, select ALL appropriate statements.

The pictured bacteria are diplococci It can be ruled out that this specimen is taxonomically found in the Staphylococcus genus. The pictured bacteria produce lipopolysaccharide (LPS), also known as endotoxin. The pictured bacteria are most likely non-acid fast.

What can you conclude about the Acid-Fast stained specimen? Based on your observation of this acid-fast stained specimen, select ALL appropriate statements.

The pictured bacteria are possibly a type of mycobacteria The pictured bacteria are acid-fast The pictured bacteria have mycolic acid in their cell walls

The following image is of a specimen that was grown on Starch agar for 8 hours. Following incubation, the plate was flooded with iodine. Based on this information, as well as your observation of the pictured sample, select all appropriate statements. Select all accurate statements regarding the presented scenario and the pictured test result.

The result is negative for starch hydrolysis. The plate was not incubated long enough; this could be a false negative.

Which of the following is the most appropriate way to carry a microscope?

The student correctly grasps the microscope arm with one hand and supports the base with the other hand. This minimizes the risk of dropping the heavy microscope during transport.

Andy performed an Ornithine Decarboxylase test on his unknown sample. The following image shows what the tube looked like following 24 hours of incubation at 35 ± 2°C. What information can Andy get from this test?

The test organism fermented glucose. The purple surface indicates that there is likely some decarboxylase activity. Andy forgot to add a layer of mineral oil to his sample so the surface of the medium became alkaline from air exposure.

What can you conclude based on your observation of the pictured Sucrose Fermentation test? Based on your observation of the pictured Sucrose Fermentation test, choose all appropriate statements.

The test organism ferments sucrose The test organism lowered the pH of the medium The test organism produced gas

What can you conclude based on your observation of the pictured Glucose (dextrose) Fermentation test? Based on your observation of the pictured Glucose (dextrose) Fermentation test, choose all appropriate statements.

The test organism is unable to ferment glucose The test organism did not produce gas

Based on your observation of the pictured Urea broth what can you conclude? Based on your observation of the pictured Urease test, select all appropriate conclusions . (Recall that an observation and a conclusion are different. An observation is simply what you see; conclusions are statements of what you learned from your observations.)

The tested bacterium does not produce urease

An unknown specimen was grown on Mannitol Salt agar (MSA). Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Mannitol Salt agar (MSA) test result.

The tested organism is likely to be coagulase positive. The media used in this test is good for growing Staphylococci. The media used in this test is differential. The media used in this test is selective.

The following is a micrograph of Trichinella spiralis larvae encysted in muscle tissue. Which of the image choices demonstrates the greatest increase in resolution?

This image has the greatest resolution because it's been properly focused. This was accomplished by adjusting the fine focus knob (the image is seen under the 10X objective). The higher resolution allows the details of the tissue and encysted larvae to be seen with greater precision.

An unknown specimen was grown on Indole media. Based on your observation of this specimen choose the appropriate statements. Select ALL appropriate statements regarding the pictured Indole test.

This is a picture of a negative test result. This test is a part of the IMViC panel of tests. This media is inoculated by stabbing the agar in the tube.

Below is a Glucose Fermentation test. Choose all appropriate statements about the Glucose Fermentation test.

This media is inoculated using an inoculating loop This test detects a fermentation reaction The test contains phenol red which turns yellow under acidic conditions The mouth of the tube should be flamed prior to inoculation and immediately before capping the tube

An unknown specimen was grown on Indole media. Based on your observation of this specimen choose the appropriate statements.

This test is a part of the IMViC panel of tests The organism tested converts tryptophan to indole. This media is inoculated by stabbing the agar in the tube. This is a picture of a positive test result.

In a compound light microscope, what is the function of the condenser?

To concentrate light on the specimen. Light diverges through the air as it exits the light source. The condenser gathers this divergent light and concentrates it on the specimen.

You performed the following tests to identify your unknown bacterial species, and obtained these results. Unfortunately, one of your culture tubes was contaminated during inoculation, which resulted in an INCORRECT result. Use the ID matrix provided to determine which of the test results listed is most likely due to this contamination, and therefore INCORRECT.

Voges-Proskauer All the test results match Enterobacter aerogenes except the Voges-Proskauer test. E. aerogenes typically gives a positive result with the Voges-Proskauer test, but your results table lists a negative result instead. This suggests that your unknown is most likely E. aerogenes, and the contaminated tube was the one used for the Voges-Proskauer test. Escherichia coli is another possibility. However, neither the indole test, nor the methyl red test gave results consistent with E. coli. E. aerogenes is a better fit.

You performed the Gram stain. You are expecting to find purple Gram-positive cocci and pink Gram-negative bacilli. Instead, you observe purple cocci but don't seem to see any bacilli. How could you explain this?

You forgot the safranin step. Safranin is used to color the Gram-negative bacteria that have been decolorized in the previous step. At the beginning of this video, you will note that the Gram-negative cells have no color. If the safranin step is omitted, then the Gram-negative cells will be colorless and difficult to see.

You performed the Gram stain on a smear. You are expecting to find purple Gram-positive bacilli and pink Gram-negative cocci. Instead, you observe pink bacilli and pink cocci. Which of the following is NOT a possible explanation for this unusual result?

You skipped the safranin step. If the safranin step were skipped, none of the cells would take up a pink color. This would not explain why the Gram-positive cells were pink instead of purple.

Immersion oil should be cleaned off an objective with which product?

lens paper Using lens paper helps ensure the lens won't be scratched during cleaning. In addition, lens paper is essentially lint-free; no fibers will be left on the lens that could hurt the objective's performance.

With respect to the image of a microscope specimen, refraction of light by the objective lens enhances __________.

magnification Magnification refers to the apparent size of the specimen when viewed through the microscope. Light bends at the convex surfaces of the objective lens, causing light rays to diverge and radiate outward. This divergence of light creates an image of the specimen that is larger than what would normally be seen by the naked eye.