Rhythmic Response of Serratia to Elevated Temperature16-hr culture werep)laced in atubecooled to 16C in a water bath. At intervals, 0.5-ml samples were removed and tested' for survival

JOURNAL OF BACTERIOLOGY, Mar., 1965Copyright a 1965 American Society for Microbiology

Vol. 89, No. 3Printed in U.S.A.

Rhythmic Response of Serratia marcescens

to Elevated TemperatureROBERT L. DIMMICK

Naval Biological Laboratory, School of Public Health, University of California, Berkeley, California

Received for publication 24 November 1964

ABSTRACTDIMMICK, ROBERT L. (University of California, Berkeley). Rhythmic response of

Serratia marcescens to elevated temperature. J. Bacteriol. 89:791-798. 1965.-Popula-tions of Serratia marcescens of varied ages and pretreatments, which had been grown ina chemically defined medium, were subjected to thermal stress at 50 to 56 C. The num-bers of survivors were plotted vs. time to form survivor curves, and the curves wereassembled to form three-dimensional models. The manner in which survivors varied asa function of age and time of heating was variable and often rhythmic. Different three-dimensional patterns were found when different inoculum for the test culture was used.Apparently some "dead" cells again produced colonies after extended heating periods(recuperation); this tendency varied with the age of the culture. Diminutive colonyforms, which produced normal colonies upon transfer, appeared and disappeared duringheating; this tendency fluctuated with age. It is suggested that survivor curves repre-sent a distribution of resistant forms within the population, and that this distributionvaries in a manner best described in terms of servomechanistic response within eachcell and within a given culture. Difficulties of attempting to relate changes in specificmolecular species to subsequent whole-cell responses are discussed.

Hansen and Riemann (1963) recently publishedan excellent review of heat resistance of bacteria,and other workers (Sykes, 1963; Postgate andHunter, 1963; King and Hurst, 1963) have pre-sented data concerned with the general field ofbacterial survival. But the variety of responsepatterns of microorganisms to elevated tempera-tures noted in these and many other papers leadsone to believe that our knowledge of the subjectis far from complete.Although it is well established that "young,"

actively growing cells are more sensitive to almostany trauma than "mature" cells, no one hasexamined the manner in which this change occursin context with the implied change in the single-hit theory (either young cells have fewer sensitivesites or the sites are more easily inactivated),especially in regard to the genetic apparatus. Thequestion is important, because the survival ofbacteria is employed as an index of disinfectantefficacy, or, in terms of decay rates, survival isused to measure implied molecular changes in theinternal structure of the cell (Webb, 1959).

In an attempt to create a model system, I haveexamined survivor curves of Serratia marcescensas a function, primarily, of test temperature andage of culture. Although the study is not com-plete, a sufficient body of data has been accumu-lated to indicate that the reaction of this species

to thermal stress is extremely complex, and iscertainly not explainable by the assumption oflogarithmic death kinetics (single-hit hypothesis)or on the basis of simple thermodynamic princi-ples.

MATERIALS AND METHODSOrganism. A strain of Serratia marcescens, la-

beled 8 UK, was obtained from the U.S. Depart-ment of Agriculture, Western Regional ResearchLaboratory. Normal cultures (i.e., those between4 and 24 hr old, in growth medium, and held atconstant growth temperature) consistently pro-duced light-red colonies about 2 mm in diameteron peptone or Blood Agar Base (BAB, D)ifco) after24 hr of incubation at 31 C, and bright-red colonieson a solid chemically defined medium (CDM)containing (per liter): dibasic ammonium citrate,2.5 g; glycerol, 5 ml; K2HPO4 , 7.8 g; MgSO4 , 0.25g; NaCl, 0.13 g (all anhydrous); and agar, 20 g.The medium was minimal; reducing the concen-tration of any component reduced the total cellyield of 1010 to 2 X 1010 viable cells per milliliter inshake-flask cultures of the liquid menstruum at31 C after 24 hr of incubation.

Assay of viability. Ten-fold serial dilutions of aculture sample were made in the same medium inwhich cells were grown or tested. A pipette havinga stainless-steel tip with an orifice calibrated todeliver 0.02 ml per drop was used to plant five dropson each of three plates of nutrient agar; the plates,

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rotated sufficiently to spread the drops slightly,were incubated at 31 C for 24 hr unless noted. Thenumber (mean of three plates) of colonies was ob-tained with the aid of an electronic colony counter(Leif and Wolochow, 1958). This technique hasbeen shown to produce replicate assays of normalcultures having a 95'1 confidenice interval of +8%.All colonies were counted regardless of size.

Cadthres and sospensions. Unless noted, a loop-ful of the organisrmi was transferred from a stockslant into 100 nml of C)AI contained in a 500-mlflask. The flask was incubated on a rotarv shakerat 31 C for 24 hr. A 1-i111 amount of the culture wastransferred to a seconid flask and inceubated foreither 10 or 24 hr (see below); 1 ml from the secondflask was planted in a third flask. Immediately,and at the noted intervals thereafter, 0.5-ml sam-ples were removed for testing. In sonme instances,Heart Infusion Broth (Difco) was used instead ofCDMI; in others, the third flask was inoculatedwith 1 ml of a 10-fold, or higher, dilution of thesuspension in the second flask rather than with 1ml directly.

Determination of solrvivor' cmrves and patterns. Awater bath was Imaintained at the required tem-perature (50 to 56 C) by means of a Thermistemp(Yellow Springs Instrument Co., Yellow Springs,Ohio) control unit. Test tubes (2.5 by 20 cm) con-taining 9.5 ml of the same mediumi in which the

109 -_

N \ 24 hr culture

52 C

Chemically Defined lMedium(see text)

107

20 40 80 100 120Time of h eat In, m ini tes

FIG. 1. Effect of age of cuilture on survivor curves

of Serr)atia marcescens dtdring thermnlal stress at 52 C.In all instances, points listed as zero heating tinmewere taken i?ninediately after sample was added toheated media, aned repr esent at least 10 sec of heating.

test organismi was suspended were immllersed in thewater bath to withini 2.5 cm1i of their tops. Afterthe medium equilibrated for 30 min, 0.5 mnl of thetest culture was carefully added, the tube wasshaken for 5 see, and a 0.5-imil saIm1ple was remnovedto a(ldilution blaink and assayed. Thereafter, at10, 20, 30, 40, 60, 90, aind 120 niii, uniless noted,0.5-mil samples were removed froeam the heatedsamiiple and assayed. Every 10-fold or every 100-fold dilution (0 to 7) was plated, as above, foreach assay. Culture samiples to be heated in thismlanner were reniioved from the third flask athourly intervals for 8 hr, and then at 10, 16, 22,and 24 hr. 'Minor variations froam the latter timesequence were somletimes made for practical pur-poses.Each assay datuni was plotted Oii seiiiilog paper

above an appropriate time scale, and a line wasdrawn through the points to create a survivorcurve. For mnodel construction, the curve wastraced on plastic sheet, the sheet was cut to shape,and sheets were mounted serially in direct rela-tionship to the culture age. Spaces between thesheets were filled in with clay to produce a three-dimensional model of best fit. The meodels werephotographed, and tracings of the photographsare presented.

RESULTSFigure 1 shows typical survivor curves from

a culture during growth. Repeated experimentsshowed that enhanced sensitivity usually oc-curred with younger cultures, together with atendency for the number of viable cells to increaseupon prolonged heating (recuperation), and thatsigmoid curves without recuperation alwaysoccurred with older cultures. It is important tonote that these recuperation peeriods never ex-ceeded 30 min; i.e., they were less than onegeneration time (47 min in CDM\ at 31 C).To test the possibility that clumping caused

the riesults to appear nonlogarithmic, culturesof several ages were filtered through membranefilters (1.5-, pore size), and some weere treatedwith nonlethal ultrasonic energy. I found essen-tially the same results as above, including evi-dence of "tailing" and periods of reculperation.

Figure 2 shows the results of assembling datain the form of three-dimensional models. Wrhencultur es weIre started from 1 0-hr-old inocula,the most pronounced p)eriods of lrecul)eration(labeled R) occuirred at about 6 hr aftei inocula-tion, regardless of the stress teml)erature (A, 13,C, an(l 1) in Fig. 2). W hen a culture started froma 24-hr inoculumil (F) was tested, no markedrecuperation period was noted, and the generallysigmoid shape of the curves was reduced. Re-peated tests confirmedI the general "hill andvalley" appearance of the models.A culture that had been stored 5 weeks at 4 C

in CDMAI was used directly as an inoculum. The

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survival pattern from the resulting culture isshown in E (Fig. 2). Slight recuperation periodswere noted, though the curves were more linearin shape than those resulting from the use of a10-hr inoculum.

It was logistically unfeasible to obtain com-plete survivor curves of a single growing cultureduring the entire 24-hr period at intervals of lessthan 1 hr. Instead, survival was assayed every20 mi during growth by sampling cells after 90min of heating. The results (Fig. 3) are equivalentto a cross-section of the survival patterns shownin Fig. 2. In Fig. 3, a distinction is made betweensamples that had countable numbers of coloniesas the result of a direct transfer to agar surfacesand those that were countable only in a dilutedsample, to show that the rapid rise and fall ofsurviving numbers included more than one orderof magnitude. Evidently, the survival patternscontained "fine structure" not apparent in Fig. 1and 2.

This "fine structure" was also demonstratedby sampling a heated culture every minute fora 30-min interval, with samples placed directlyon agar surfaces and the order of single droplets(ca. one per 10 see) on each plate recorded. Acurve, from data obtained by a second techniciantesting the same culture, based on samples atless frequent intervals, followed the general trendof the rapid samples (Fig. 4). The periodicityof the fluctuations appeared to lengthen as afunction of time of heating. The first 12 platesobtained in this manner contained mixtures ofwhite diminutive (see below), red diminutive,and red normal colonies; the next 8 plates con-tained red diminutive and red normal colonies;and the last 10 contained red normal coloniesonly. The number of colonies per drop on a singleplate varied as much as 1:50 and sequentially, ina manner suggesting a trend rather than a randomchange.To ascertain whether a cold shock before heat-

ing would cause temporal changes, 10 ml of a16-hr culture were p)laced in a tube cooled to 16 Cin a water bath. At intervals, 0.5-ml sampleswere removed and tested' for survival at 52 C.The results (Fig. 5) show that survival fluctuatedas a function of time of caoling in a manner bestdescribed as a damped wave.

Identical inocula were placed in CDMI and inHeart Infusion Broth (Difco), and samples ofdifferent ages were tested with both CD_M andBAB agar as assay media. The shapes of survivorcurves of the two cultures were sometimes differ-ent, and the two sets of curves, based on colonynumbers found on CI)M or BAB plates, varied(Fig. 6). The number of colonies formed on B3ABagar, divided by the number found on CDiM,

FIG. 2. Three-dimensional survival patterns ofSerratia marcescens during thermal stress: (A) 50 C,10-hr inoculum, third pass; (B) 52 C, 10-hr inocu-lum, third pass; (C) 54 C, 10-hr inoculum, thirdpass; (D) 56 C, 10-hr inoculum, third pass; (E)52 C, inoculum stored 5 weeks at 4 C, CDM medium,direct transfer; (F) 52 C, 24-hr inoculum, third pass;(R) position of most evident recuperation period.

G ro.' tIh t iri e, h r

FIG. 3. Survival of Serratia marcescens after 90min at 50 C as a function of age of culture.

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from the same dilution (plating ratio) of normalcultures was 1.05 with a standard error of 0.05;no deviation greater than 1.1 or less than 0.8 wasnoted. Plating ratios as high as 700 and as low as

0 10 20 30 40 50 60Time of heating, minutes

FIG. 4. Comparison of rapid sequential samplingto routine sampling procedure duiring thermal stressof Serratia marcescens.

Grown in heart infusion broth

Grow

I hr old

i ,I-

n in Bunting'

4 hr old

I /d

s broth

, 7 hr old

I\I\\l

I\l

Time of heating,CD agar plates

0.1 were often found Nith injured cells, the num-ber tending to increase, then decrease, as a func-tion of time of heating.The influence of the level of inoculum (104 to

107 bacteria per milliliter) on the "resistance" offully grown cultures was tested by first determin-ing the incubation time required for each cultureto progress 2 hr into the stationary phase. Byuse of this information, duplicate cultures 2 hrinto the stationary phase were tested at 52 C for

10 20Time of pre-treatment, min

FIG. 5. Influence of time of pretreatnment at 16 Con the number of surviving cells of Se7rratia marce-scens stressed at 52 C.

10 hr otd -13 hr old 24 hr old

10 hr otd 13 hr old 24 hr old

minutes (0 to 120)---- Blood agar base plates

FIG. 6. Influence of age of culture, growth medium, and sampling medium on thermal death curves ofSerratia marcescens.

E- 105

Ca.

VM 103

L-

E> 107

105

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survival (Fig. 7). Cultures inoculated with theleast number of cells (hence, older cultures attime of test) were the most resistant, althoughthe number of cells tested was the same. Allwere more sensitive than cultures of the same agesstarted from the usual 108 bacteria (see Fig. 6).

Samples from cultures less than 16 hr old thathad been heated more than 10 min but less thanabout 90 min usually produced numerous diminu-tive colony forms on both types of media. Thepopulation of colonies varied from those justvisible to those of normal size. The smallercolonies were usually white after 24 hr of incuba-tion; most became red after 48 hr, and some in-creased in size. Numerous transfers of thesecolonies to fresh agar yielded no evidence ofmutations; i.e., all new colonies appeared normal(pseudoauxotrophy; Zamenhof, 1960); and whensurvivors were recultivated in liquid medium andtested, their progeny evinced no enhanced resist-ance. Growth curves of subcultures of heatedsamples often showed extended lag times andreduced growth rates compared with normalcultures, although some finally attained a normalrate of growth. But after cultures had beenheated approximately 100 min, the survivingcells produced colonies that were uniform in sizeand color when the plates were incubated for 24hr. Reversion to "normal" colony productionwas especially evident during the recuperationperiod: the number of cells that produced normalcolonies after 120 min of heating and 24 hr ofincubation was sometimes 1,000 times the numberthat produced normal colonies after 30 min ofheating and 48 hr of incubation.

In addition, instances of a gross lack of agree-ment between the expected numbers of coloniesarising from sequential dilutions were oftenobserved during the 15- to 100-min period, aphenomenon I prefer to term "incoherence,"because the true extent of viability was no longerdeterminable. Usually these instances involveda lack of agreement between the direct-transfersample (zero dilution) and the first 10-fold dilu-tion. Sometimes the plates containing a zerodilution produced no colonies, whereas platescontaining the next higher dilution contained toomany colonies to count; sometimes the reversewas observed. No instances of this kind wereobserved in thousands of assays of cultures thathad not been treated in some way generally con-sidered to be detrimental to the life of the cell.When heat-killed cells were added to living cellsbefore heating, no differences in either the timeof occurrence or the extent of incoherence wasobserved.Twenty duplicate platings from several dilu-

tions of a single sample of a 6-hr-old culture

1 2 3 1 2 3Time of heating, hours

FIG. 7. Influence of level of inoculumn on thermaldeath curves of Serratia marcescens in the early sta-tionary phase. Duplicate survivor curves were madefor each test.

grown in CDM and heated for 10 min at 52 Cwere made on two types of media. All plates wereimmediately incubated. At hourly intervals onTrypticase Soy Agar plates, and at 2-hr intervalson CDM plates, duplicate sets of each mediatype were treated by spreading the surfacemoisture with sterile dally rods. The plates werereincubated and examined 24 and 48 hr afterthey had been dallied. Colony numbers, bothtotal and diminutive, increased as a function oftime after plating (Fig. 8). Although there wasno increase of numbers of colonies on undalliedCDM plates as a result of additional incubation,the number increased on plates dallied duringthe first 10 hr. The number of diminutive coloniesapproximately doubled, and then approachedzero when rapid growth started, indicating, Ibelieve, that the diminutive characteristic waspassed to at least one generation. Since the totalnumber of colonies also increased and the colonysizes were heterogeneous, there must have beena heterogeneity of initial generation times greaterthan in "normal" populations, indicating eithera variety of injured sites or a variety of responsesto a similar injury in all cells. In this instance,approximately 10 times the number of colonieswere formed on CDMI plates compared with thenumber on Trypticase Soy Agar plates (platingratio ca. 0.1). There were too few colonies, in-cluding diminutive colonies, on Trypticase SoyAgar plates in the lowest dilution tested to besignificant for the purpose of this experiment,although I have shown the total number in Fig.

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DIMMICK

al No. Colonies \V

ypticase Soy)

4 8 12 16Time in hrs that plates were streaked

after inoculationFIG. 8. Change in colony number as a result of

streaking plates at intervals after they had beeninoculated from the same dilution sampled from a

6-hr culture of Serratia marcescens heated to 52 C for10 min. Two types of media were used: Blood AgarBase and chemically defined medium (CDM1).

8 for comparison. Colonies that arose on Trypti-case Soy Agar were almost all normal in sizeafter 48 hr of incubation.

DIscussIoN

The object of these exploratory experimentswas to obtain a notion of factors that mightinfluence the general form (shape) of survivorcurves without initial reference to "rate" theoriesor to any specific molecular species in the cell. Inother words, if one discards preconceived con-

cepts and assumes that a suspension of bacteriais a "black box" system (Rescigno and Segre,1961) about which little is actually known, but a

system that nonetheless has measurable "output"characteristics (colony number, size, growthrate, temperature response, etc.), one may adjustthe "input" (environment), measure the output,and hope to gain knowledge of internal mecha-nisms by deductive means. This concept is incontrast to that embodied in many current studiesof cellular physiology that prefer to take thebox apart and hope that structural propertiesretain those characteristics present in the intactassembly; both avenues obviously yield usefulinformation.On the basis of these data, several general

statements can be made about suspensions of S.marcescens cells. First, the apparently irregularaction of mild heat on S. marcescens is in generalagreement with the findings presented by White(1953) and Lemcke and White (1959) for Strepto-

coccus faecalis and for Escherichia coli. Second,cell populations did not die logarithmically. Infact, and despite the weighty arguments of WVood(1956), the most reasonable conclusion is that ofJordan, Jacobs, and Davies (1947) and Vas andProszt (1957); i.e., a survival curve represents akind of cumulative distribution of the "resist-ance" of the individual cells in the pl)oulation.Third, the distribution varied, sometimes rhyth-mically, as a function of culture age or of pre-treatment. The inoculum for a given cultureinfluenced the extent of rhythm. Responses suchas these were predicted by Hinshelwood (1951).Fourth, the capacity of some cells to form coloniesafter varied periods of stress was influenced bythe act of sampling, age of culture, and a temporalfunction of a change in environment prior to test.Fifth, ability to recuperate from such lethalconditions, again depending on age, was mostoften enhanced by enriched plating medium, butsometimes enriched medium discouraged thegrowth of many cells. Harris (1963) reviewedthis phenomenon in relationship) to several otherspecies. Sixth, the duration of all these responsesvaried from seconds to hours, often in a mannerresembling a damped wave phenomenon or aself-regulating system seeking a point of stability.The concept of the cell as a complex, dynamic

servomechanism has been l)resented, or at leastimlplied, by Hinshelwood (1951), Heinmetz(1960), Elsasser (1960), and Chance (1961).Ideas presented by 1Iora (1963) and by Deanand Hinshelwood (1963) add emphasis to thisconcel)t. Apparently overlooked in these discus-sions is the fact that the "output" of one cellcan be the "input" of another. Thus, in a cultureof sufficient numbers, groups of cells may acttogether (a biophase; Ilerret, 1960), and theculture may l)ossess some of the same servo-mechanistic properties as the cell, but with longercycles. There is evidence that the culture as awhole might have acted autonomously during andafter this type of stress, and it is evident thatindividual cells changed in some way duringstress according to conditions imposed upon themby the culture as a whole before stress.

I suggest that the death of most cells resultedfrom a lack of over-all control (i.e., an imbalancedstate), and that this occurred in a variety of ways.Of course, real logarithmic death rates can occurin a population many lethal, quantitized phe-nomena could kill a cell if that cell is dependenton, and is "hit" in, a v-ital site; i.e., if a one-to-onerelationship exists. But other, less lethal stressescan so damage a cell that it either does not grow,or does not grow immediately in the environmentimposed for pUrlposes of riecultivation. Conversely,the finding of a linear decay on a semilog plot

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need not imply that a monomolecular reaction isoccurring, or that, having demonstrated anexponential relationship in one instance, it mustapply to all instances. The problem is to dis-tinguish between the two kinds of damage. Time-dependent repair mechanisms that interpose"noise" between the lethal event and the meas-urement of that event, as well as differences inindividual cell structure and response coupledwith cell-to-cell interaction, cause the interpre-tation of every survival curve to be suspect,especially if "decay constants" are to be esti-mated. The presence of a colony is proof of life;the absence of a colony is not proof that deathhad occurred.

Harrison and Lawrence (1963) and Hess andShon (1962) attributed periods of enhancedviability, or of "tailing" of survivor curves as aresult of stress, to the growth of selected mu-tants or variants. My data show instances offluctuation of viable numbers too rapid to beconsidered a result of cell replication duringstress, and fluctuations during growth too re-peatable (qualitatively) to be entirely randomin nature. I found no instances of enhancedresistance of populations grown from resistantcells. The progeny from small white coloniesproduced red colonies of normal size after asingle transfer. Apparently, some type of non-lethal injury (e.g., lengthened generation time)can be passed to a limited number of generationswithin a colony.

Obviously, the majority of these data rest onthe behavior of a small fraction of the totalpopulation tested, but this is the fraction uponwhich concepts of sterility or epidemiology areusually based. Whether these resistant cellsare true "mutants" or not is of no consequenceto the present argument, because survivors areundoubtedly members of the species and there-fore represent overall species capabilities; it isthe transient nature of their p)resence and absencethat is important. Certainly, there was no selec-tion pressure to encourage varieties resistant toheat during culture growth. Recuperation, oreven incoherence in specific instances, might beexplained on the basis of thermal renaturation ofdeoxyribonucleic acid (D)NA; 'Marmur and Doty,1961), or by assuming random repair of geneticinjury, but this does not explain the rapid andconsistent rise and decline of either resistance orrecuperative capacity dur ing gorowth of theculture.

Attempts to relate survival mechanisms tospecific biochemical changes, observed fromstudies of enough cells to be analyzed by availa-ble techniques (for example, analysis of DNAcontent), may yield information pertinent to

average behavior, but would not reveal individualcellular capabilities. Conversely, colony forma-tion (as a phenotypic measurement) is a whole-cell response. Death of some cells could be at-tributed to metabolic deficiencies induced by thestress, but not in instances where more colonieswere found on chemically defined medium thanon enriched medium.

Again, differences in cellular resistance seemedto be more closely associated with time-de-pendent organization than with a given molecularcomplement or with genetic accidents, althoughthese attributes are not easily separated andmeasured in large populations. There exists noproven technique for distinguishing live cellsfrom dead ones other than ability to reproduce,and the argument is circular, because the defini-tion of life, with regard to bacteria, impliesreproduction. An explanation of the behavior ofmicrobial cultures in terms of servomechanismsmay also be circular, but the theory can betested by available techniques, mathematicalmodels can be constructed, and computer tech-niques can be applied (Sugita, 1961).One such highly informative model was sug-

gested by Goodwin (1963). In discussing theepigenetic system, he pointed out that probablyno oscillation would be observed in bacteriabecause of the small numbers of a given speciesof messenger ribonucleic acid (RNA) per cell;he added, "There is one rather comforting ob-servation which we can make at this point, how-ever, and that is that no rhythmic or cyclic be-haviour has ever been observed in bacteriaanalogous to the tidal, diurnal, lunar and otherrhythms which are such an obtrusive feature ofbehaviour in higher organisms, from the protozoaup." I suggest that two types of rhythmic be-havior, with age and with environmental shift,have now been demonstrated in at least onespecies of bacteria, and that we ought to beaware of this possibility in other species underother test conditions.

ACKNOWLEDGMENTS

I wish to acknowledge the intellectual stimulusand initial suggestions of Carl Lamanna, and alsothe help of Stephen A. Dunn and other membersof the Naval Biological Laboratory technical staffwho performed the major part of these labors.

This investigation was supported by the Officeof Naval Research under a contract with the Re-gents of the University of California.

LITERATU-RE CITED

CHANCE, B. 1961. Control characteristics of en-zyme. Cold Spring Harbor Symp. Quant. Biol.16 :289-299.

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ELSASSER, W. M. 1960. Quanta and the concept oforganismic law. J. Theoret. Biol. 1:27-58.

COODWIN, B. C. 1963. Temporal organization incells. Academic Press, Inc., New York.

hIANSEN, N. H., AND H. RIEMANN. 1963. Factorsaffecting the heat resistance of nonsporing or-ganisms. J. Appl. Bacteriol. 26:314-333.

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HARRISON, A. P., AND F. R. LAWRENCE. 1963.Phenotypic, genotypic, and chemical changesin starving populations of Aerobacter aerogenes.J. Bacteriol. 85:742-750.

HEINMETZ, F. 1960. An analysis of the concept ofcellular injury and death. Int. J. Radiat. Biol.2:341-352.

HESS, G. E., AND M. SHON. 1962. Selection ofthermostable Serratia marcescens from logarith-mic-phase cultures as a means for inducingsynchronous growth. J. Bacteriol. 83:781-784.

HINSHELWOOD, C. N. 1951. Decline and death ofbacterial populations. Nature 167:666-669.

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LEIF, W. R., AND H. WOLOCHOW. 1958. An elec-tronic relay colony counter for virulent or-ganisms. Amer. J. Med. Technol. 24:6-7.

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MARMUR, J., AND P. DOTY. 1961. Thermal renatur-ation of deoxyribonucleic acids. J. Mol. Biol.3:585-594.

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RESCIGNO, A., AND G. SEGRE. 1961. The product-precursor relationship. J. Theoret. Biol. 1:498-513.

SUGATA, M. 1961. The switching circuit logicallyor functionally equivalent to a system of bio-chemical reactions. J. Phys. Soc. Japan 16:737-740.

SYKES, G. 1963. The phenomenon of bacterialsurvival. J. Appl. Bacteriol. 26:287-294.AS, K., AND G. PROSZT. 1957. Observation of theheat destruction of spores of Bacillus cereus.J. Appl. Bacteriol. 21:431-441.

WEBB, S. J. 1959. Factors affeeting the viabilityof airborne bacteria. I. Bacteria aerosolizedfrom distilled water. Can. J. Microbiol. 5:649-669.

WHITE, H. R. 1953. The heat resistance of Strepto-coccus faecalis. J. Gen. MIicrobiol. 8:27-37.

WOOD, T. II. 1956. Lethal effects of high and lowtemperatures on unicellular organisms. Ad-vanee. Biol. Med. Phys. 4:119-165.

ZAMENHOF, S. 1960. Effects of heating dry bacteriaand spores on their phenotype and genotype.J. Nat. Acad. Sci. U.S. 46:101-105.

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The only other gram-negative organisms to showa large zone of clearing were one strain of Proteusancd one of Alcaligenes. Several other Proteuisstrains showed small equivocal zones, but, 92%of the Proteuts strains showed absolutely no ac-tivity by thi,s method. None of the strains ofEsclericlhia coli, Klebsiella, or Aerobacter showedevidence of an extracellular deoxvribonuclease.The )resence of an extracellular deoxvribo-

nuclease, under the l)articular assay conditionsemployed, in all strains of Serratia tested butonly in a rare strain of other gram-negativebacilli provides an additional simple laboratorytest for the identification of this organism.

This investigation was supported by PublicHealth Service grant 5TI Al 215-02 from the Na-tional Institute of Allergy and Infectious Diseases.

ERRATA

Regulatory Mechanisms in the Biosynthesis ofIsoleucine and Valine

I1. Identification of Two Operator GenesT. RAMAKRISHNAN AND EDWARD A. ADELBERG

Departnten t of Mlicrobioloyy, Yale Un iversity, New Haven, Connecticut

Volume 89, no. 3, p. 659, co(. 1, linle 2: change "sensitivity to acid" to "insensitivity to acid."

Regulatory Mechanisms in the Biosynthesis ofIsoleucine and Valine

III. Map Order of the Structural Genes and Operator GenesT. RAMAKRISHNAN AND EDWARD A. ADELBERG

Departtme It of Mficrobiology, Yale University, Nvew Haven, Con necticulit

V'olume 89, no. 3, ). 662, col. 1, line 14 of "Results": change AB1514 to AB12070. Line 19 of "Re-sults": chan,e AB1206 to A.O1305. Table 1: for strain AB1255 add str-17 or 9 or 8; add the genotypefor strain A1B2070: ilvE12, nietEf,46, try-3, his-4, thi-1, gal-2, lac-1 or 4, nwal-1, intl-1, str-S or 9, T6r-3,pro-2, ara-9.

Rhythmic Response of Serratia marcescens toElevated Temperature

ROBERT L. DIMMICKNaval Biological Laboratory, School of Puiblic Health, University of California, Berkeley, California

Volume 89, no. 3, page 791, col. 2, line 12 of "AMaterials and AMethods": Clhange "K2HPO4, 7.8 g" to"K2HPO4, 3.9 g."

295N'OTES