Buoyancy-driven fluid flow generated by bacterial metabolism and its proposed relationship to increased bacterial growth in space

Previous investigations have reported that bacterial growth increases in space flight; however, the underlying physical mechanisms responsible for these changes have not been fully determined. As bacteria consume nutrients, they excrete by-products whose presence can influence the onset of exponential growth and affect final cell population density. It is assumed that these metabolic processes create a reduced-density fluid zone and/or a solute gradient around each cell. On Earth, this density difference may result in local buoyancy-driven convection of the excreted by-products. The absence of convection and sedimentation in the lowgravity space flight environment, however, can be expected to alter the fluid dynamics surrounding the cells by limiting transport to diffusion only. Based on this biophysical model, it was hypothesized that acceleration affects the lag phase duration and final cell concentration of suspended bacterial cultures in a predictable, non-linear manner, due to the resultant changes incurred in the extracellular fluid composition. Eight experiments at various levels of acceleration consistently supported this hypothesis, resulting in predictable growth kinetics. In additional experiments, macroscopic plumes of fluid were observed and analyzed rising from metabolizing bacterial cultures. If similar fluid dynamics were found to occur on a microscopic level, it would help explain how acceleration affects bacterial growth kinetics. REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 222024302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) 18-03-2005 2. REPORT TYPE Journal Article 3. DATES COVERED (From To) Spring 2005 5a. CONTRACT NUMBER N/A 5b. GRANT NUMBER N/A 4. TITLE AND SUBTITLE Buoyancy-driven fluid flow generated by bacterial metabolism and its proposed relationship to increased bacterial growth in space 5c. PROGRAM ELEMENT NUMBER N/A 6. AUTHOR(S) Robert B. Brown, David M. Klaus and Paul Todd 5d. PROJECT NUMBER N/A 5e. TASK NUMBER N/A 5f. WORK UNIT NUMBER N/A 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER Department of Astronautics 2354 Fairchild Dr. US Air Force Academy, CO 80840 N/A 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) N/A N/A 11. SPONSOR/MONITOR’S REPORT NUMBER(S) N/A 12. DISTRIBUTION / AVAILABILITY STATEMENT A – Approved for public release; distribution is unlimited. 13. SUPPLEMENTARY NOTES Supported in part by NASA Cooperative Agreement NCC8-242 14. ABSTRACT Previous investigations have reported that bacterial growth increases in space flight; however, the underlying physical mechanisms responsible for these changes have not been fully determined. As bacteria consume nutrients, they excrete by-products whose presence can influence the onset of exponential growth and affect final cell population density. It is assumed that these metabolic processes create a reduced-density fluid zone and/or a solute gradient around each cell. On Earth, this density difference may result in local buoyancy-driven convection of the excreted by-products. The absence of convection and sedimentation in the low-gravity space flight environment, however, can be expected to alter the fluid dynamics surrounding the cells by limiting transport to diffusion only. Based on this biophysical model, it was hypothesized that acceleration affects the lag phase duration and final cell concentration of suspended bacterial cultures in a predictable, non-linear manner, due to the resultant changes incurred in the extracellular fluid composition. Eight experiments at various levels of acceleration consistently supported this hypothesis, resulting in predictable growth kinetics. In additional experiments, macroscopic plumes of fluid were observed and analyzed rising from metabolizing bacterial cultures. If similar fluid dynamics were found to occur on a microscopic level, it would help explain how acceleration affects bacterial growth kinetics. 15. SUBJECT TERMS Bacterial Growth, Fluid Flow, Buoyancy-driven fluid flow, Growth Kinetics 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON LtCol Robert B. Brown a. REPORT U b. ABSTRACT U c. THIS PAGE U N/A 20 19b. TELEPHONE NUMBER (include area code) (719) 333-4110 Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18 INSTRUCTIONS FOR COMPLETING SF 298 1. REPORT DATE. Full publication date, including day, month, if available. Must cite at least the year and be Year 2000 compliant, e.g. 30-06-1998; xx-06-1998-, xx-xx-1998. 2. REPORT TYPE. State the type of report, such as final, technical, interim, memorandum, master's thesis, progress, quarterly, research, special, group study, etc. 3. DATES COVERED. Indicate the time during which the work was performed and the report was written, e.g., Jun 1997 Jun 1998; 1-10 Jun 1996; May Nov 1998; Nov 1998. 4. TITLE. Enter title and subtitle with volume number and part number, if applicable. On classified documents, enter the title classification in parentheses. Ba. CONTRACT NUMBER. Enter all contract numbers as they appear in the report, e.g. F33615-86C-5169. 5b. GRANT NUMBER. Enter all grant numbers as they appear in the report, e.g. AFOSR-82-1234. 5c. PROGRAM ELEMENT NUMBER. Enter all program element numbers as they appear in the report, e.g. 61101A. 5d. PROJECT NUMBER. Enter all project nurnbers as they appear in the report, e.g. 1F665702D1257; ILIR. 5e. TASK NUMBER. Enter all task numbers as they appear in the report, e.g. 05; RF0330201; T4112. 5f. WORK UNIT NUMBER. Enter all work unit numbers as they appear in the report, e.g. 001; AFAPL30480105. 6. AUTHOR(S). Enter name(s) of person(s) responsible for writing the report, performing the research, or credited with the content of the report. The form of entry is the last name, first name, middle initial, and additional qualifiers separated by commas, e.g. Smith, Richard, J, Jr. 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES). Self-explanatory. 8. PERFORMING ORGANIZATION REPORT NUMBER. Enter all unique alphanumeric report numbers assigned by the performing organization, e.g. BRL-1234; AFWL-TR-85-4017Vol-21-PT-2. 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES). Enter the name and address of the organization(s) financially responsible for and monitoring the work. 10. SPONSOR/MONITOR'S ACRONYM(S). Enter, if available, e.g. BRL, ARDEC, NADC. 11. SPONSOR/MONITOR'S REPORT NUMBER(S). Enter report number as assigned by the sponsoring/ monitoring agency, if available, e.g. BRL-TR-829; -21 5. 12. DISTRIBUTION/AVAILABILITY STATEMENT. Use agency-mandated availability statements to indicate the public availability or distribution limitations of the report. If additional limitations/ restrictions or special markings are indicated, follow agency authorization procedures, e.g. RD/FRD, PROPIN, ITAR, etc. Include copyright information. 13. SUPPLEMENTARY NOTES. Enter information not included elsewhere such as: prepared in cooperation with; translation of; report supersedes; old edition number, etc. 14. ABSTRACT. A brief (approximately 200 words) factual summary of the most significant information. 15. SUBJECT TERMS.