Microbial maintenance: a critical review on its quantification

Abstract

Microbial maintenance is an important concept in microbiology. Its quantification, however, is a subject of continuous debate, which seems to be caused by (1) its definition, which includes nongrowth components other than maintenance; (2) the existence of partly overlapping concepts; (3) the evolution of variables as constants; and (4) the neglect of cell death in microbial dynamics. The two historically most important parameters describing maintenance, the specific maintenance rate and the maintenance coefficient, are based on partly different nongrowth components. There is thus no constant relation between these parameters and previous equations on this subject are wrong. In addition, the partial overlap between these parameters does not allow the use of a simple combination of these parameters. This also applies for combinations of a threshold concentration with one of the other estimates of maintenance. Maintenance estimates should ideally explicitly describe each nongrowth component. A conceptual model is introduced that describes their relative importance and reconciles the various concepts and definitions. The sensitivity of maintenance on underlying components was analyzed and indicated that overall maintenance depends nonlinearly on relative death rates, relative growth rates, growth yield, and endogenous metabolism. This quantitative sensitivity analysis explains the felt need to develop growth-dependent adaptations of existing maintenance parameters, and indicates the importance of distinguishing the various nongrowth components. Future experiments should verify the sensitivity of maintenance components under cellular and environmental conditions.

Figure 1

Literature compilation of correlation between measured specific maintenance rates a and maximum relative growth rates μ_max for various microbial species ( ж [1]; [10]; × [21];○ [22]; [23]; — [26]; [29]; [33]; [34]; [36]; [43]; [44]; [58]; [59]; + [60]).

Figure 2

Comparison between the relationship between maintenance coefficient m and specific maintenance rate a calculated according to Eq. (9) and the original equation in Pirt [38] for various μ values with Y_G=0.6 g C_x g⁻¹ C_s.

Figure 3

Relationship between the substrate concentration S and the net relative growth rate μ while accounting for maintenance in different ways, i.e., by a specific maintenance rate (a), maintenance coefficient (m), or a minimum substrate concentration (S_min). Kinetic parameters were obtained for growth on 3-Chlorobenzoate [54].

Figure 4

Responses of the overall maintenance m_tot, depicted as planes, calculated by the conceptual model to variation in relative growth rate μ_r with μ_max=0.5 h⁻¹ and relative death rate d for different combinations of physiological maintenance m_p and growth yield Y_G: (a) m_p=0.01 g C_s g⁻¹ C_x and Y_G=0.6 g C_x g⁻¹ C_s; (b) m_p=0.01 g C_s g⁻¹ C_x and Y_G=0.5 g C_x g⁻¹ C_s; (c) m_p=0.10 g C_s g⁻¹ C_x and Y_G=0.6 g C_x g⁻¹ C_s; (d) m_p=0.10 g C_s g⁻¹ C_x and Y_G=0.5 g C_x g⁻¹ C_s; (e) m_p=0.50 g C_s g⁻¹ C_x and Y_G=0.6 g C_x g⁻¹ C_s; (f) m_p=0.50 g C_s g⁻¹ C_x and Y_G=0.5 g C_x g⁻¹ C_s.