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Figure 1: Voges-Proskauer test on Enterobacter aerogenes and Escherichia coli. FIG. 1. A positive Voges-Proskauer test on Enterobacter aerogenes (left) and a negative Voges-Proskauer test on Escherichia coli (right). The positive result given by Enterobacter aerogenes is indicated by the red color change after the addition of Barritt's A and Barritt's B reagents. The negative result given by Escherichia coli is indicated by the lack of color change after the addition of the reagents. (Mary G. Miller, Southeastern Louisiana University, Hammond, LA)

Figure 2: Methyl red test on Escherichia coli and Enterobacter aerogenes. FIG. 2. A positive methyl red test on Escherichia coli (left) and a negative methyl red test on Enterobacter aerogenes (right). The positive result given by Escherichia coli is indicated by the red color after the addition of methyl red reagent. The negative result given by Enterobacter aerogenes is indicated by the lack of color change after the addition of methyl red. (Mary G. Miller, Southeastern Louisiana University, Hammond, LA)

Figure 3: Methyl red test on Enterobacter aerogenes and Escherichia coli. FIG. 3. A positive methyl red test on Escherichia coli (right) and a negative methyl red test on Enterobacter aerogenes (left). The positive result given by Escherichia coli is indicated by the red color after the addition of methyl red reagent. The negative result given by Enterobacter aerogenes is indicated by the lack of color change after the addition of methyl red. (Tasha L. Sturm, Cabrillo College, Aptos, CA)

Figure 4: Methyl red test on Escherichia coli and Klebsiella pneumoniae. FIG. 4. A positive methyl red test on Escherichia coli (tubes 1 and 2) and a negative methyl red test on Klebsiella pneumoniae (tube 3). The positive result given by Escherichia coli in the positive control (tube 1) and the test (tube 2) is indicated by the red color after the addition of methyl red reagent. The negative result given by Klebsiella pneumoniae (tube 3) is indicated by the lack of color change after the addition of methyl red. (Shashidhar Vishwanath, Kasturba Medical College, Karnataka, India)

Figure 5: Voges-Proskauer test on Enterobacter aerogenes and on Kocuria rhizophila. FIG. 5.  A positive Voges-Proskauer test on Enterobacter aerogenes (left) and a negative Voges-Proskauer test on Kocuria rhizophila (right). The positive result given by Enterobacter aerogenes is indicated by the brown-red color change after the addition of Barritt's A and Barritt's B reagents. The negative result given by Kocuria rhizophila is indicated by the lack of color change after the addition of the reagents. (Julie Torruellas Garcia, Nova Southeastern University, Fort Lauderdale, FL)

Figure 6: Methyl red test on Proteus mirabilis, Escherichia coli, Enterobacter cloacae and Klebsiella pneumoniae. FIG. 6. Methyl red test. The positive result given by Proteus mirabilis and Escherichia coli is indicated by the red color after the addition of methyl red reagent. The negative result given by Enterobacter cloacae and Klebsiella pneumoniae is indicated by the lack of color change after the addition of methyl red. An uninoculated tube is also shown. The methyl red reagent was added after a 40-hour incubation at 37°C. (Jackie Peltier Horn, Houston Baptist University, Houston, TX)

Figure 7: Voges-Proskauer test on Enterobacter aerogenes, Klebsiella pneumoniae, Escherichia coli and Proteus mirabilis. FIG. 7. Voges-Proskauer test. The positive result given by Enterobacter aerogenes and Klebsiella pneumoniae is indicated by the red color change after the addition of Barritt's A and Barritt's B reagents. The negative result given by Escherichia coli and Proteus mirabilis is indicated by the lack of color change after the addition of Barritt's A and Barritt's B reagents. An uninoculated tube is also shown. Barritt's A and Barritt's B reagents were added after a 40-hour incubation. (Jackie Peltier Horn, Houston Baptist University, Houston, TX)

Figure 8: Methyl red–Voges-Proskauer test on Escherichia coli. FIG. 8. A methyl red–Voges-Proskauer (MR-VP) test done on Escherichia coli. The image depicts the results after a 48-hour incubation at 37°C. Tube A shows a positive result for the methyl red test indicated by the red color after the addition of methyl red reagent. Tube B shows a negative result for the Voges-Proskauer test indicated by a lack of color change after the addition of Barritt's A and Barritt's B reagents. (Anne Hanson, University of Maine, Orono, ME)

Figure 9: Methyl red–Voges-Proskauer test on Enterobacter aerogenes. FIG. 9. A methyl red–Voges-Proskauer (MR-VP) test done on Enterobacter aerogenes. The image depicts the results after a 48-hour incubation at 37°C. Tube A shows a negative result for the methyl red test indicated by the lack of color change after the addition of methyl red reagent.  Tube B shows a positive result for the Voges-Proskauer test as indicated by a brown-red color change after the addition of Barritt's A and Barritt's B reagents. (Anne Hanson, University of Maine, Orono, ME)

Figure 10: Methyl red test on Escherichia coli and Enterobacter aerogenes. FIG. 10. A positive methyl red test on Escherichia coli (left) and a negative methyl red test on Enterobacter aerogenes (right). The positive result given by Escherichia coli is indicated by the red color after the addition of methyl red reagent. The negative result given by Enterobacter aerogenes is indicated by the lack of change after the addition of methyl red. (Sylvia F. McDevitt, Skidmore College, Saratoga Springs, NY)

Figure 11: Voges-Proskauer on Enterobacter aerogenes and Escherichia coli. FIG. 11. A positive Voges-Proskauer test on Enterobacter aerogenes (left) and a negative Voges-Proskauer test on Escherichia coli (right). The positive result given by Enterobacter aerogenes is indicated by the red color change after the addition of Barritt's A and Barritt's B reagents. The negative result given by Escherichia coli is indicated by the lack of color change after the addition of Barritt's A and Barritt's B reagents. (Sylvia F. McDevitt, Skidmore College, Saratoga Springs, NY).

Figure 12: Voges-Proskauer test on Hafnia alvei. FIG. 12. Temperature effect on Voges-Proskauer test of Hafnia alvei. Cultures of H. alvei were grown in MR-VP broth at 30°C (right tube) or 37°C (left tube) for 24 hours before the test was performed. The culture grown at 30°C is Voges-Proskauer positive, as indicated by the red color on the top of the culture, whereas the culture grown at 37°C is negative as indicated by the lack of color change. (Sylvia McDevitt, Skidmore College Saratoga Springs, NY)