Impact of Antibiotic Spray on the Taxonomic Diversity of Citrus Bacterial Communities

Andrew McAuley

Authors:  Andrew McAuley, Marie de Gracia Coquerel, Sydney Cabana, Nabil Chowdhury, Dr. Marina Ascunce

Faculty Mentor:  Dr. Marina Ascunce

College:   College of Agricultural and Life Sciences


The citrus industry in Florida and worldwide is been seriously affected by citrus greening or Huanglongbing (HLB) disease. The causal agent, Candidatus Liberibacter asiaticus (CLas), is an unculturable bacterium that it is transmitted by an invasive insect, the citrus psyllid, upon feeding on citrus leaves. As crop losses reach up to 90%, Florida growers are desperate, resulting in the spraying of human antibiotics streptomycin (STR) and oxytetracycline (OTC) on citrus groves to treat HLB. Because soil microbiota represents a natural source of antibiotic resistance genes in the environment, the ongoing and widespread antibiotic applications in citrus-growing areas of Florida are expected to select for antibiotic resistance bacteria (ARB) and possibly for horizontal transfer of antibiotic-resistance genes (ARGs) within the microbial community. We hypothesize that after antibiotic application on citrus trees, there will be an increase in the frequency and diversity of ARBs and ARGs in the citrus environment. Sampling of the roots, leaves, and soil of citrus trees in fields that have had antibiotic applications and a field that did not have antibiotic applications were done. DNA extractions, PCRs, and high-throughput 16S amplicon sequencing were conducted. An initial assessment of the microbial taxonomic diversity will be discussed.

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15 Responses
  1. Marina Ascunce

    Thank you for visiting our poster. We are holding an open zoom virtual presentation from 3 to 4pm, please join us using the link below.

    Join Zoom Meeting

    Meeting ID: 584 594 161
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    If you are interested in our research topic, please email Dr. Ascunce ( and also visit our companion poster about an undergraduate course that we are teaching on microbial communities:
    (also available for zoom from 3 to 4pm).
    Dr. Ascunce

  2. Erica Goss

    Hi Andrew, Nice poster. Do you think there are different bacteria in different groves? Did you see an effect of the antibiotics on colony number in the same grove over time?

    1. Andrew McAuley

      Good afternoon Dr. Goss,

      Based on what we saw on the plates in regards to colony diversity, it looked like there were differences in the types of colonies present on the plates with the antibiotic application versus the organic groves. We need to do the DNA analysis to confirm.

      We have not looked into changes in colony diversity in the same grove over time but that would be a very interesting question to look into next.


    2. Andrew McAuley

      Good afternoon Dr. Goss,

      Based on the colony counts we did, it looks like there are different types of colonies present on the plates from the samples of the organic grove versus the antibiotic spray groves. We need to do the DNA analysis to confirm the identity of the bacteria present.

      We have not yet looked at the bacterial diversity over time in the same grove but that would be a very interesting question to look into next.


  3. Emily Miller

    Hi Andrew! Great poster!
    What kind of effect do you expect an increase in ARBs and ARGs to have on the citrus groves?

    1. Andrew McAuley

      Thanks for your question Emily!

      We expect the ARBs and ARGs to increase antibiotic resistance as a whole in the groves with the antibiotic sprays. This can cause long term problems since the antibiotic applications would not be as effective on the fields.

      This may also lead to the evolution of new pathogens if those bacteria with the antibiotic resistance genes also gain any other virulence genes.


  4. Jade Monteiro

    Hi Andrew! Would the spraying of the human antibiotics STR and OTC even be effective in treating HLB disease since it is not seen in humans? Thanks!

    1. Andrew McAuley

      Hi Jade,

      The reason that the antibiotic spray contains streptomycin and oxytetracycline is that they are both antibiotics that are effective against gram negative bacteria, which C. liberibacter is.

      Both antibiotics work through blocking protein synthesis in the bacterial cell.

      Even though they are classified as human antibiotics, they can still be applied to other systems and be effective in killing bacteria. The system in this case is the citrus trees.


  5. Jade Monteiro

    Hi Andrew! Would the human antibiotics STR and OTC even be effective in treating HLB disease since it is not a disease humans encounter? Thanks!

  6. Caroline McMillan

    Hey Andrew! Great job with the poster and presentation! I know the study was cut short, but if it had been able to continue, how would you expect the results to turn out? Would it continue on the same trends shown?

    1. Andrew McAuley

      Hi Caroline,

      Once the data is able to be analyzed properly, we expect the trend to be followed that the field sprayed with antibiotics will have a lower overall bacterial diversity when compared to the organic field. We are not sure which species will be present on each. Only the data analysis will tell!


  7. Ryan Husain

    Hi Andrew,
    How could citrus growers combat harmful diseases without creating a population of ARGs and ARBs?