Effects of pH on the Growth of Staphylococcus aureus, Streptococcus bovis, and Escherichia coli species

Pam Shelton

Biology Undergraduate, Health Science Concentration

Tennessee Technological University

Introduction/Background Staphylococcus aureus

• “responsible for a wide range of human disease, including septicemia; endocarditis and pneumonia; and wound, bone and joint infection (Feil et al 2003).”

• “associated with nosocomial infections (Caiazza and O’Toole 2003).”

Introduction (cont)

Streptococcus bovis (Group D)• Group B Streptococcus is “the most common

cause of bacterial infection in the newborn (Curtis et al 2003).”

• Streptococcus bovis (Group D Streptococcus) is important to understand because it is “capable of causing fulminant neonatal sepsis or meningitis that is indistinguishable clinically from that caused by group B streptococcus (Gavin et al 2003).”

Introduction (cont)

Escherichia coli• Associated with food products• “presence of feces on meat products is a

primary source of foodborne pathogens, such as Escherichia coli 0157:H7 (Ashby 2003).”

Introduction (cont)

All cause disease if present in large amounts or immune system is compromised

Learning about growth allows for development of ways to limit growth

Objective/Hypothesis Hypothesis: Various bacteria require

different pH levels in order to grow properly. each organism will have an optimal pH

specific for that particular organism. most organism will have similar ranges in

which they prefer to grow.

Objective: To determine the pH range and optimal pH for the growth of Staphylococcus aureus, Streptococcus bovis, and Escherichia coli

Materials and Methods

Set of tubes containing 3 mL Tryptic Soy Broth (TSB) were treated with varying amounts of HCl or NaOH to adjust pH from 1 to 14 • pH measured with pH paper• Similar to method used by Kelly et al (2003)

Tubes were inoculated with one of the organisms and incubated at 36 °C for 48 hrs

Amount of growth was measured and recorded

Materials and Methods

Another set of tubes were prepared with the same method except with a more narrow pH range

Tubes were inoculated and incubated with the previous method

Three trials of the procedure were done for each organism• Temperature of incubation, incubation time, and

broth composition (except pH) were constant in each trial

Materials and Methods

Results and Discussion

S. aureus preferred a pH range of 4.0 – 9.25 in trials 1and 2 and 4.5 – 9.25 in trial 3

Optimal pH of S. aureus was around pH 7 This agrees, for the most part, with the work previously done

Feeherry et al (2003).• “S. aureus growth occurs in the pH range 4.5 to 9.3 (7.2

optimum).” There is slight disagreement with the lowest (4.0) and highest

(9.25) values of the pH range found in this experiment. This is probably due to experimental error.

• The pH paper used is not as accurate as a pH meter • It can be assumed that in trial 1 and 2, the range is within the

expected 4.5 – 9.3.

Results and Discussion

Staphylococcus growth

0

0.5

1

1.5

2

2.5

1 2 3 4 4.25 4.5 4.75 5 6 7 8 9 9.25 9.5 9.75 10 11 12 13 14

pH

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Results and Discussion

S. bovis only grew very close to pH 7 This agrees Chamkha et al (2003)

“strain B7…grew optimally at pH 7”

Results and Discussion

Streptococcus growth

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 6.25 6.5 6.75 7 7.25 7.5 7.75 8 9 10 11 12 13 14

pH

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Results and Discussion

E. coli grew equally well in the range of pH 7 to pH 9. These results disagree with the previous work

E. coli is capable of growing in a “pH range of 2.5 – 9.3 (Vinneras et al 2003).”

even with experimental error taken into consideration, these results are too extreme to go unnoticed

Explanation: To survive in extremely acidic conditions, E. coli has evolved

three adaptive acid resistance strategies to maintain internal pH (Gong et al 2003).”

“all strains succumbed to pH 2.5 in the absence of arginine (Gong et al 2003).”

This growth broth did not contain arginine. So, it was incapable of acid tolerance.

Results and Discussion

Escherichia coli growth

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 6.25 6.5 6.75 7 8 9 9.25 9.5 9.75 10 11 12 13 14

ph

am

ou

nt

of

gro

wth

Results and Discussion

As expected in my hypothesis, the growth of microorganisms is effected by the pH of their surrounding environment.

I also expected: most organism would have similar ranges in which they

prefer to grow. each organism would have an optimal pH specific for that

particular organism. This was disproved through the results of this experiment.

ranges varied greatly depending on the particular organism the optimal pH was closely centered around 7 for most

organisms

Summary and Conclusions

pH effects growth of microorganisms Organisms have a wide variety of

preferred pH ranges Most organisms optimal pH is centered

closely around 7 Information about growth of

microorganisms can help prevent the spread of disease

Questions?