The Uptake and Release of Amino Acids by Staphylococcus aureus at Mid-Exponential and Stationary Phases and Their Corresponding Responses to Changes in Temperature, pH and Osmolality

Abstract

Staphylococcus aureus is an important pathogen that is associated with nosocomial infections, as well as food poisoning. This bacterium is resistant to antimicrobial agents and can survive in a wide range of environmental conditions. The aim of this study was to measure the uptake and release of amino acids by S. aureus at midexponential and stationary phases of growth following exposure to a combination of conditions including variations in temperature, pH and NaCl. Bacterial cells were grown up to mid-exponential and stationary phases in tryptic soy broth (TSB), where the supernatants were collected for analyses of amino acids to determine the uptake and release characteristics. The uptake/release of amino acids was estimated by subtracting the initial levels of the free amino acids in the media from those measured at mid-exponential and stationary phases of growth. When cells were grown at ideal conditions, the analyses revealed that significant uptake of amino acids had occurred by stationary phase compared with the mid-exponential phase. A substantial release of valine and tyrosine into the external media was observed by cells at stationary phase. At both phases, the uptake and release patterns were significantly different between cells grown under ideal control conditions, when compared with those grown under various combinations of sub-optimal environmental conditions. The analyses of the supernatants harvested from controls and treatment groups at exponential phase indicated that the total uptake of amino acids was reduced approximately five times by cells grown with addition of 2.5% NaCl or with pH6 at 35 • C, and 2-fold by cells grown at pH8 at 35 • C. However, the final quantities of amino acids taken up by cells grown to stationary phase did not significantly alter between control and treated samples. Valine was found to be the most abundant amino acid that was significantly released into the media at stationary phase by both control and treated samples. It was evident that diverse environmental conditions resulted in differential patterns of amino acid uptake and release during adaptation to designated conditions.

Riboswitches as Drug Targets for Antibiotics

“…For example, while antibiotic compounds targeting the lysC riboswitch regulating lysine biosynthesis were discovered [ 105 ], the riboswitch is not considered a viable target because many bacteria harbour isozymes that are not regulated by the riboswitch [ 106 ]. Further, many pathogens can sequester lysine from the host, rendering inhibition of lysine biosynthesis ineffective [ 107 , 108 ]. Lafontaine and colleagues demonstrated that their most promising guanine riboswitch ligand PC1 ( Figure 5 b) only acts against clinically relevant bacteria that have guaA under riboswitch regulation [ 101 ].…”

Metabolic Profiles of Clinical Strain of Staphylococcus aureus to Subtle Changes in the Environmental Parameters at Different Phases of Growth

“…Earlier studies have shown that S. aureus altered many genes in order to combat changes in pH and hence facilitate survival [25][26][27][28] . An identical study showed that amino acid and protein composition of S. aureus were remarkably altered following exposure to either pH6 or pH8 combined with lower temperature of 35°C 6,29 . Another study determined significant changes in the fatty acid composition and cell sizes of Staphylococcus lugdunensis in response to alterations in environmental conditions including changes in pH and temperature 8 .…”

“…A recent study exhibited that hydrogen peroxide combined with other environmental factors such as altered pH and temperature led to very different metabolic profiles of a clinical isolate of S. aureus 10 , suggested these alterations in amino acids were essential for survival under changes in the environmental conditions. Moreover, a new study revealed that S. aureus taken up and released different levels of amino acids following growth in altered environmental conditions 29 .…”

“…The synthesis of proteins in high concentrations could be a mechanism to enhance the adaptation and hence the survivability of S. aureus at complex environment present in stationary phase. S. aureus was shown to consumed different levels of amino acids in response to various environmental conditions at stationary phase 29 , suggesting that different amino acid uptakes were required for adaptation under various temperature and pH.…”

Analysis of Cytoplasmic and Secreted Proteins of Staphylococcus aureus Revealed Adaptive Metabolic Homeostasis in Response to Changes in the Environmental Conditions Representative of the Human Wound Site

“…Several studies have investigated how S. aureus and Staphylococcus lugdunensis adapt to changes in the environmental conditions, mimicking wound site conditions: pH 6–8, temperature 35–39 °C, and adding NaCl to increase osmolality by up to 5%. Staphylococcal species adapted to these conditions by inducing considerable alterations in the composition of membrane fatty acids and cytoplasmic amino acids, as well as changes in ribosomal proteins and cell sizes [ 6 , 19 , 20 ]. The adaptive changes also demonstrated in S. aureus under a similar design were consistent with the formation of metabolic phenotypes characteristic of the prevailing environmental conditions [ 21 ].…”