applied sciences Review A Brief Recap of Microbial Adhesion and Biofilms Spyridon Achinas 1,*, Nikolaos Charalampogiannis 2 and Gerrit Jan Willem Euverink 1 1 Faculty of Science and Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 2 Department of Urology, SLK Kliniken am Gesundbrunnen, Am Gesundbrunnen 20-26, 74078 Heilbronn, Germany * Correspondence: [email protected] Received: 24 June 2019; Accepted: 9 July 2019; Published: 12 July 2019 Abstract: Food and beverage industries operate their production units under stringent hygiene standards to verify high-quality products. However, the presence of biofilms can cause hygienic problems in the industries in the case of pathogenic organisms. Microorganisms can form biofilms, which are resistant to cleaning and disinfection. Microorganisms in biofilms are closely packed in a matrix that acts as a barrier to cleaning and disinfection. Biofilms are observed in processing equipment and open surfaces, resulting in food safety problems or weakening of production efficiency. This review provides a recap of the biofouling process, including the production mechanisms and control techniques of microbial adhesion. Microbial adhesion and colonization are the sine qua non of the establishment of bacterial pathogenesis and this report focuses on their prevention. Keywords: microorganisms; bacterial growth; biofilm; adhesion; control 1. Introduction Food, dairy, maritime, water systems, oil, paper, opticians, dentistry, hospitals, and domestic households are several examples of industries that are affected by the biofilms [1–8]. The occurrence of biofilms leads to serious problems, e.g., product spoilage, reduced production efficiency, corrosion, unsightliness, infection, pipe blockages, and equipment failure [9,10]. A biofilm is defined as an organized aggregate of microorganisms living within an extracellular polymeric matrix that they produce. This is irreversibly attached to either a material surface or a living surface, which cannot be easily removed due to its complex structure [11,12]. This complex layer of extracellular polymeric substances (EPS) protects them from antibiotics, disinfectants, and dynamic environments [13,14]. The intercellular communication within the biofilm rapidly stimulates the up and down-regulation of gene expression. This property enables temporary adaptations, such as phenotypic variation and the ability to survive in nutrient deficient conditions. These pitfalls and challenges create an industrial interest in developing materials, methods, and tools that can prevent or remove the formation of biofilms. The food and beverage industries operate their production units under stringent hygiene standards to verify high-quality products. However, the presence of biofilms can cause hygienic problems in the case of pathogenic organisms. Biofilms consist of complex microbial communities that adhere to the surfaces fencing processes, like cleaning and disinfection [15]. Biofilms are observed in processing equipment and open surfaces, resulting in food safety problems or a weakening of the efficiency of the production lines [16]. It is important to control the formation of biofilms to avoid corrosion and contamination. This review provides a brief of the biofouling process, including the production mechanisms and control of microbial adhesion. Appl. Sci. 2019, 9, 2801; doi:10.3390/app9142801 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2801 2 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 14 2. Bacterial Growth TheThe growth growth of of a a bacterium bacterium isis defineddefined asas the incr increaseease in in cells cells number number rather rather than than the the cell cell size. size. There are certain requirements for bacterial growth and factors that influence the growth process. There are certain requirements for bacterial growth and factors that influence the growth process. Temperature, pH, osmotic pressure, mixing, and oxygen concentration are the main factors Temperature, pH, osmotic pressure, mixing, and oxygen concentration are the main factors influencing influencing bacterial growth. There are also nutritional requirements, besides these factors. The bacterial growth. There are also nutritional requirements, besides these factors. The replication process replication process is not through mitoses and meiosis, but through a process that is known as binary is not through mitoses and meiosis, but through a process that is known as binary fission. The DNA fission. The DNA of a singular bacterium will be replicated during binary fission (Figure 1). ofAfterwards, a singular bacteriumthe DNA will will be be divided, replicated and during the cell binarywall and fission the membrane (Figure1). will Afterwards, form a transverse the DNA willseptum. be divided, The transverse and the cell septum wall will and be the completely membrane formed, will form which a transverse will enable septum. cells separation, The transverse thus septumgenerating will be new completely bacterial cells. formed, The time which interval will enable required cells for separation, the cell division thus is generating one pivotal new parameter bacterial cells.in bacterial The time growth, interval which required is known for as the generation cell division time. is The one generation pivotal parameter time depends in bacterial on the type growth, of whichorganism is known and the as generationenvironmental time. conditions. The generation time depends on the type of organism and the environmental conditions. FigureFigure 1. 1.This This imageimage displaysdisplays the binary binary fission fission process process [17]. [17 ]. TheThe bacterial bacterial growth growth consists consists of of fourfour growthgrowth phases,phases, which which are are depicted depicted in in Figure Figure 2.2 In. In the the first first phase,phase, the the so-called so-called lag lag phase, phase, the the bacteria bacteria mustmust physiologically adapt adapt to to the the culture culture conditions conditions and and growthgrowth begins begins after after a period a period of time. of time. The densityThe densit in whichy in which the cells the are cells distributed are distributed is another is importantanother factor.important Broadly, factor. exoenzymes Broadly, exoenzymes and nutrients and nutrients are shared are in shared proximity; in proximity; however, however, if the if density the density is low, thisis couldlow, this lead could to a dilutionlead to a ofdilution these materials of these mate [16].rials The [16]. latter The results latter in results slow cellin slow growth cell andgrowth transition and to thetransition log phase. to the Depending log phase. onDepending the conditions, on the theconditions, lag phase the may lag takephase only may minutes take only or severalminutes hours. or several hours. The time length of the lag phase depends on the type of medium. The time length of the lag phase depends on the type of medium. The lag phase is followed by the log or exponential phase. In this phase, the cells are in an The lag phase is followed by the log or exponential phase. In this phase, the cells are in an optimum optimum growth state and they perform binary fission. During the log phase, there is a logarithmic growth state and they perform binary fission. During the log phase, there is a logarithmic increase in increase in cell number. The slope of this line is the bacterial growth. The mathematical description cell number. The slope of this line is the bacterial growth. The mathematical description of the bacterial of the bacterial growth in the log phase can be described while using the following equation: growth in the log phase can be described while using the following equation: = (1) δN = µN (1) where N is defined as the cell number, t as the timeδt and µ as the specific growth rate constant (1/time). This leads to the following expression for the number of present cells (exponential growth), where N is defined as the cell number, t as the time and µ as the specific growth rate constant (1/time). which is given by: This leads to the following expression for the number of present cells (exponential growth), which is given by: = (2) µt where N0 is the number of cells at t = 0. N = N0e (2) where N0 is the number of cells at t = 0. Appl. Sci. 2019, 9, 2801 3 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 14 FigureFigure 2. 2.This This graph graph shows shows thethe relationrelation between time time an andd the the growth growth of of bacteria bacteria in ina alaboratory laboratory environmentenvironment [17 [17].]. InIn the the stationary stationary phase, phase, the the bacterial bacterial growthgrowth slows down down until until it it reaches reaches maximum maximum growth growth and and thethe logarithmic logarithmic cell cell number number remains remains constant. constant. The The mathematical mathematical expressionexpression ofof thethe stationarystationary phase is expressedis expressed as: as: δN =0= 0 (3) (3) δt ThereThere is is a balancea balance between between cellcell divisiondivision and cell death death and and there there is is no no actual actual bacterial bacterial growth. growth. ThisThis is causedis caused by by the the depletion depletion of of critical critical nutrientsnutrients or the accumulation accumulation of of waste waste product. product. Some Some of ofthe the cellscells can can lyse lyse nutrients nutrients from from the the dead dead cellscells (endogenous metabolism). metabolism). This This process process occurs occurs throughout