You know that (1) both

e. coli and salmonella have the gapa gene, (2) that all salmonella must have the apee and inva genes to make people sick, and (3)

e. coli do not have the apee or inva. given those facts, how would you interpret a pcr test that comes out positive for gapa, positive for apee, but negative for inva, using a bacteria that is making people sick?

Complete question

It has to be E. coli because it is positive for gapA It can't be Salmonella, because it is negative for the invA marker but still makes people sick It has to be used because it is positive for the apeE marker

1. It can't be Salmonella, because it is negative for the invA marker but still makes people sick

A combination of positive results from the multiplex PCR inva, apee and gapa is used to definitively identify Salmonella.  Pathogenic, or disease causing Salmonella is definitely not present, as inva is required for it to be pathogenic. The test did not detect the inva sequence, thus it is truly negative for this particular pathogen.

2. It has to be E. coli because it is positive for gapA

E. coli may be present, as gapa was detected- a presumptive positive. However, this may need to be definitively determined through further methods of sample analysis, such as 2D-gel electrophoresis. apee may belong to another type of bacteria present within the initial sample, or there may be sample contamination

Explanation:

Polymerase chain reactions, PCRs are a form of nucleic acid amplification testing NAAT that exploit the mechanism of transcription by using a thermostable DNA polymerase. These require a sample of genetic material such as RNA or DNA; specific regions of the gene sequence are targeted for replication by primers.  In the presence of these specific gene sequences, the primers make billions of copies of the sequences.

However, if these gene sequences are absent, the primers are not capable of identifying and amplifying the sequence. This reaction is highly specific. Positives obtained have a high chance of being true positives and negatives have a high chance of being true negatives .

Pathogens or infectious agents that are capable of causing disease i.e. making people sick. Both E. coli and Salmonella are genuses of enteric bacteria capable of causing disease via fecal contamination. Common symptoms include

abdominal crampsvomitingfeverdiarrhea

This test would include primers for the detection of each sequence: gapa, apee, inva. Salmonella's inva was not detected, thus it is not present.

Further steps may include a 2 D gel  electrophoresis- here an electrical current is utilized to separate bands of DNA within the sample. This should correspond with an expected DNA  size in base pairs or bp  for E.coli- this should be determined by running the sample in the gel with a positive control, containing genetic material for E coli, and a negative control, of purified water to determine contamination.

The correct answer is 1